Fix clippy::needless_pass_by_ref_mut warning

```
warning: this argument is a mutable reference, but not used mutably
   --> src/process.rs:125:23
    |
125 |     pub(crate) fn run(&mut self) -> Result<()> {
    |                       ^^^^^^^^^ help: consider changing to: `&self`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut
    = note: `-W clippy::needless-pass-by-ref-mut` implied by `-W clippy::nursery`
    = help: to override `-W clippy::nursery` add `#[allow(clippy::needless_pass_by_ref_mut)]`

warning: this argument is a mutable reference, but not used mutably
   --> src/process.rs:142:35
    |
142 |     pub(crate) fn run_with_output(&mut self) -> Result<Output> {
    |                                   ^^^^^^^^^ help: consider changing to: `&self`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut
```

Author: taiki-e

src/main.rs

@@ -480,7 +480,7 @@ fn exec_on_package(
     match kind {
         Kind::Normal => {
             // only run with default features
-            return exec_cargo(cx, id, &mut line, progress, keep_going);
+            return exec_cargo(cx, id, &line, progress, keep_going);
         }
         Kind::Each { .. } | Kind::Powerset { .. } => {}
     }
@@ -510,7 +510,7 @@ fn exec_on_package(
         // run with all features
         // https://github.com/taiki-e/cargo-hack/issues/42
         line.arg("--all-features");
-        exec_cargo(cx, id, &mut line, progress, keep_going)?;
+        exec_cargo(cx, id, &line, progress, keep_going)?;
     }
 
     if !cx.no_default_features {
@@ -524,7 +524,7 @@ fn exec_on_package(
 
     if !cx.exclude_no_default_features {
         // run with no default features if the package has other features
-        exec_cargo(cx, id, &mut line, progress, keep_going)?;
+        exec_cargo(cx, id, &line, progress, keep_going)?;
     }
 
     match kind {
@@ -567,7 +567,7 @@ fn exec_cargo_with_features(
 ) -> Result<()> {
     let mut line = line.clone();
     line.append_features(features);
-    exec_cargo(cx, id, &mut line, progress, keep_going)
+    exec_cargo(cx, id, &line, progress, keep_going)
 }
 
 #[derive(Default)]
@@ -643,7 +643,7 @@ impl FromStr for LogGroup {
 fn exec_cargo(
     cx: &Context,
     id: &PackageId,
-    line: &mut ProcessBuilder<'_>,
+    line: &ProcessBuilder<'_>,
     progress: &mut Progress,
     keep_going: &mut KeepGoing,
 ) -> Result<()> {
@@ -667,7 +667,7 @@ fn exec_cargo(
 fn exec_cargo_inner(
     cx: &Context,
     id: &PackageId,
-    line: &mut ProcessBuilder<'_>,
+    line: &ProcessBuilder<'_>,
     progress: &mut Progress,
 ) -> Result<()> {
     if progress.count != 0 && !cx.print_command_list && cx.log_group == LogGroup::None {

src/process.rs

@@ -121,7 +121,7 @@ impl<'a> ProcessBuilder<'a> {
 
     /// Executes a process, waiting for completion, and mapping non-zero exit
     /// status to an error.
-    pub(crate) fn run(&mut self) -> Result<()> {
+    pub(crate) fn run(&self) -> Result<()> {
         let status = self.build().status().with_context(|| {
             process_error(format!("could not execute process {self:#}"), None, None)
         })?;
@@ -138,7 +138,7 @@ impl<'a> ProcessBuilder<'a> {
 
     /// Executes a process, captures its stdio output, returning the captured
     /// output, or an error if non-zero exit status.
-    pub(crate) fn run_with_output(&mut self) -> Result<Output> {
+    pub(crate) fn run_with_output(&self) -> Result<Output> {
         let output = self.build().output().with_context(|| {
             process_error(format!("could not execute process {self:#}"), None, None)
         })?;
@@ -155,7 +155,7 @@ impl<'a> ProcessBuilder<'a> {
 
     /// Executes a process, captures its stdio output, returning the captured
     /// standard output as a `String`.
-    pub(crate) fn read(&mut self) -> Result<String> {
+    pub(crate) fn read(&self) -> Result<String> {
         let mut output = String::from_utf8(self.run_with_output()?.stdout)
             .with_context(|| format!("failed to parse output from {self:#}"))?;
         while output.ends_with('\n') || output.ends_with('\r') {

src/rustup.rs

@@ -160,7 +160,7 @@ pub(crate) fn install_toolchain(
 }
 
 fn minor_version() -> Result<u32> {
-    let mut cmd = cmd!("rustup", "--version");
+    let cmd = cmd!("rustup", "--version");
     let output = cmd.read()?;
 
     let version = (|| {