Do not hash absolute sysroot path into stdlib crates metadata. (rust-…

…lang#14951)

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.

(Some context: 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 whatever, but at least it won't change based on
the rustc path anymore)

Overview of the fix:
- For libstd crates, the metadata hash now contains the path relative to
the sysroot, instead of the absolute path.
- The absolute 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

@@ -604,11 +604,20 @@ fn compute_metadata(
 
     METADATA_VERSION.hash(&mut shared_hasher);
 
+    let ws_root = if unit.is_std {
+        // SourceId for stdlib crates is an absolute path inside the sysroot.
+        // Pass the sysroot as workspace root so that we hash a relative path.
+        // This avoids the metadata hash changing depending on where the user installed rustc.
+        &bcx.target_data.get_info(unit.kind).unwrap().sysroot
+    } else {
+        bcx.ws.root()
+    };
+
     // 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())
+        .stable_hash(ws_root)
         .hash(&mut shared_hasher);
 
     // Also mix in enabled features to our metadata. This'll ensure that