Use path APIs instead of MAIN_SEPARATOR splitting in app/build.rs (#14563)

## Description
`get_build_profile_name()` in `app/build.rs` derived the Cargo profile
name by reading `OUT_DIR` as a plain string and splitting it on
`std::path::MAIN_SEPARATOR`. Choosing the separator by hand is brittle,
so this replaces the string split with real path handling: `OUT_DIR`
becomes a `PathBuf` and the profile directory is resolved with
`ancestors().nth(3)` + `Path::file_name()`.

Behavior is unchanged for the standard `OUT_DIR` layout
(`.../<profile>/build/<crate>-<hash>/out` -> `<profile>`), and the
path-based version additionally tolerates a trailing separator, which
the old split did not.

Per the requester, no comments and no tests were added, and the change
is limited to that one function.

## Linked Issue

[APP-5072](https://linear.app/warpdotdev/issue/APP-5072/use-path-apis-instead-of-main-separator-string-splitting-in-appbuildrs)

## Testing
Testing-exempt: the requester explicitly asked for no new tests, and
this is a behavior-preserving refactor of a build-script helper.

Verification performed instead:
- Equivalence check: compiled and ran a standalone harness with the old
and new implementations side by side over representative `OUT_DIR`
values
(`/code/core/target/cli/build/my-build-info-9f91ba6f99d7a061/out`,
`.../debug/build/warp-abc123/out`, `.../release-lto/build/warp-1/out`) —
both return the same profile name (`cli`, `debug`, `release-lto`). With
a trailing separator the old split returns `build` while the new
path-based version still returns `cli`.
- `cargo fmt -p warp -- --check` passes.
- `cargo check -p warp` passes — the build script compiles and runs
(`target/debug/build/warp-*/output` produced).
- `cargo clippy -p warp --all-targets --tests -- -D warnings` passes.
- No UI surface (build script only), so no computer-use verification
applies.

- [ ] I have manually tested my changes locally with `./script/run`

## Agent Mode
- [x] Warp Agent Mode - This PR was created via Warp's AI Agent Mode

Originating thread:
https://warpdev.slack.com/archives/C0BDQDW8V5E/p1785445025743429

CHANGELOG-NONE

<!-- factory-agent:
{"source":"factory-agent","task_id":"APP-5072","task_source":"linear","task_url":"https://linear.app/warpdotdev/issue/APP-5072/use-path-apis-instead-of-main-separator-string-splitting-in-appbuildrs","oz_run_id":"019fb544-9a9c-7b19-bf65-bee1c2de61bc","repo":"warpdotdev/warp"}
-->

Co-authored-by: Andy <andy@warp.dev>

Author: warp-agent-staging[bot]

app/build.rs

@@ -218,12 +218,14 @@ fn generate_channel_config_if_needed(target_family: &str, target_os: &str) {
 fn get_build_profile_name() -> String {
     // The profile name is always the 3rd last part of the path (with 1 based indexing).
     // e.g. /code/core/target/cli/build/my-build-info-9f91ba6f99d7a061/out
-    env::var("OUT_DIR")
-        .expect("OUT_DIR must be set")
-        .split(std::path::MAIN_SEPARATOR)
-        .nth_back(3)
+    let out_dir = PathBuf::from(env::var_os("OUT_DIR").expect("OUT_DIR must be set"));
+    out_dir
+        .ancestors()
+        .nth(3)
+        .and_then(Path::file_name)
         .expect("could not get profile name")
-        .to_string()
+        .to_string_lossy()
+        .into_owned()
 }
 
 fn add_features(target_family: &str, target_os: &str) {