rust-1.88: set rust.lld to false to fix FTBFS

Upstream have changed to no longer unconditionally build LLD in
bootstrap, instead only building it if a system LLVM was not in use.
The defaults were not updated before 1.88 was released to reflect this,
however, so the boostrap will still expect `rust-lld` to have been
built.  This change is recommended by upstream in
rust-lang/rust#143076 (comment)

See also rust-lang/rust#143255 which will fix
this for post-1.88 releases (which shouldn't matter for us, as we'll
still have this configuration change in place).

Author: OddBloke

rust-1.88.yaml

@@ -92,6 +92,11 @@ pipeline:
         --set="target.${ARCH}.musl-root=/usr" \
         --set="target.${ARCH}.crt-static=false"
 
+      # Configure the bootstrap process not to use Rust's LLD (as we use our
+      # system LLVM); see
+      # https://github.com/rust-lang/rust/issues/143076#issuecomment-3018803649
+      sed -i '/^\[rust\]$/a lld=false' bootstrap.toml
+
   - runs: |
       cd rustc-${{package.version}}-src
       sed 's|deny(warnings,|deny(|' -i src/bootstrap/src/lib.rs