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

@@ -63,6 +63,7 @@ pipeline:
       cat > config.toml <<EOF
       [rust]
       deny-warnings = false
+      lld = false
       EOF
 
       ./configure \