Limit to one job on mingw builders

This is another attempt to work around
rust-lang#108227.

By limiting to one link job, we should be able to avoid file name
clashes in mkstemp().

src/ci/run.sh

@@ -65,7 +65,7 @@ RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set dist.compression-profile=best"
 # This is an attempt to fix the spurious build error tracked by
 # https://github.com/rust-lang/rust/issues/108227.
 if isWindows && [[ ${CUSTOM_MINGW-0} -eq 1 ]]; then
-    RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set llvm.link-jobs=4"
+    RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set llvm.link-jobs=1"
 fi
 
 # Only produce xz tarballs on CI. gz tarballs will be generated by the release