Fix cross-compilation of build.rs for aarch64 target

Build scripts may run on the host architecture, not the target. When
cross-compiling from x86_64 to aarch64, #[cfg(target_arch = "aarch64")]
evaluates to false in the build script, causing KRUN_EDK2_BINARY_PATH
to not be set at compile time:

  error: environment variable `KRUN_EDK2_BINARY_PATH` not defined at compile time
     --> src/vmm/src/builder.rs:104:44
      |
  104 | static EDK2_BINARY: &[u8] = include_bytes!(env!("KRUN_EDK2_BINARY_PATH"));
      |                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use CARGO_CFG_TARGET_ARCH environment variable instead, which reflects
the actual target architecture regardless of the host.

Signed-off-by: Adam Ford <adam.ford@anodize.com>

Author: Adam Ford

src/vmm/build.rs

@@ -1,6 +1,5 @@
 fn main() {
-    #[cfg(target_arch = "aarch64")]
-    {
+    if std::env::var("CARGO_CFG_TARGET_ARCH").as_deref() == Ok("aarch64") {
         let edk2_binary_path = std::env::var("KRUN_EDK2_BINARY_PATH").unwrap_or_else(|_| {
             format!(
                 "{}/../../edk2/KRUN_EFI.silent.fd",