Do not hash absolute sysroot path into build-std metadata.

stdlib crates get a SourceId which has an absolute path pointing into the sysroot. This makes the metadata hash change depending on where you've installed Rust. This is causing problems because the different hashes snowball into the optimizer making different decisions which ends up changing the binary size. At work we're working with embedded devices with little flash storage so it often happens that a binary builds locally and then fails to fit in flash in CI, just because CI has installed rustc to a different path. Improving binary size is *not* a goal of this PR, after the fix the size will be randomly big or small but at least it won't change randomly anymore.

The path is still hashed into the fingerprint (not the metadata) so moving the rustc installation triggers a rebuild. This ensures stdlib crates are rebuilt when upgrading nightly versions.

The rustc version is still hashed into the metadata as usual, so upgrading Rust releases (not nightly versions) does cause a metadata change.

Repro of the bug:

```
$ git clone https://github.com/embassy-rs/embassy --branch cargo-nondet-repro
$ cd embassy/
$ cd examples/nrf52840
$ RUSTUP_HOME=~/.rustup1 cargo build --release --bin wifi_esp_hosted
....
    Finished `release` profile [optimized + debuginfo] target(s) in 13.33s
$ llvm-size target/thumbv7em-none-eabi/release/wifi_esp_hosted
   text	   data	    bss	    dec	    hex	filename
 114500	     80	  48116	 162696	  27b88	target/thumbv7em-none-eabi/release/wifi_esp_hosted
$ RUSTUP_HOME=~/.rustup2 cargo build --release --bin wifi_esp_hosted
....
    Finished `release` profile [optimized + debuginfo] target(s) in 9.64s
$ llvm-size target/humbv7em-none-eabi/release/wifi_esp_hosted
   text	   data	    bss	    dec	    hex	filename
 114272	     80	  48116	 162468	  27aa4	target/thumbv7em-none-eabi/release/wifi_esp_hosted
```

src/cargo/core/compiler/build_runner/compilation_files.rs

@@ -604,12 +604,18 @@ fn compute_metadata(
 
     METADATA_VERSION.hash(&mut shared_hasher);
 
-    // Unique metadata per (name, source, version) triple. This'll allow us
-    // to pull crates from anywhere without worrying about conflicts.
-    unit.pkg
-        .package_id()
-        .stable_hash(bcx.ws.root())
-        .hash(&mut shared_hasher);
+    if unit.is_std {
+        // SourceId for build-std crates is a full absolute path inside the sysroot.
+        // Don't hash it to avoid the metadata hash changing depending on where the user installed rustc.
+        unit.pkg.name().hash(&mut shared_hasher);
+    } else {
+        // Unique metadata per (name, source, version) triple. This'll allow us
+        // to pull crates from anywhere without worrying about conflicts.
+        unit.pkg
+            .package_id()
+            .stable_hash(bcx.ws.root())
+            .hash(&mut shared_hasher);
+    }
 
     // Also mix in enabled features to our metadata. This'll ensure that
     // when changing feature sets each lib is separately cached.