Fix Diff-Check job for subtree sync PRs

[ytmimi]

rustfmt currently has a runtime dependency on the sysroot. So when we build a standalone rustfmt binary we need to set LD_LIBRARY_PATH so each rustfmt binary knows where to find it's dependencies.

When running our Diff-Check job to test PRs for breaking changes it's often the case that both the master rustfmt binary and the feature branch binary have the same runtime dependencies so we only need to set LD_LIBRARY_PATH once.

However, when running the diff-check job against a subtree sync PR that assumption doesn't hold. The subtree sync PR bumps the required toolchain used to build rustfmt and therefore the binary that gets built for the subtree sync PR has a different runtime dependency than the master rustfmt binary.

Now we set LD_LIBRARY_PATH twice to account for this potential difference.

[ytmimi]

I Ran a Diff-Check job on my fork of rustfmt using the fix_diff_check_job_for_subtree_sync_prs branch. Here's what the additional log output looks like:

compiling with cargo 1.75.0-nightly (8eb8acbb1 2023-10-17)
Building rustfmt from src
Switched to a new branch 'fix_diff_check_job_for_subtree_sync_prs'
branch 'fix_diff_check_job_for_subtree_sync_prs' set up to track 'feature/fix_diff_check_job_for_subtree_sync_prs'.
Building feature rustfmt from src
Runtime dependencies for rustfmt -- LD_LIBRARY_PATH: /home/runner/.rustup/toolchains/nightly-2023-10-22-x86_64-unknown-linux-gnu/lib:/home/runner/.rustup/toolchains/nightly-2023-10-22-x86_64-unknown-linux-gnu/lib:
RUSFMT_BIN rustfmt 1.7.0-nightly (041f113 2023-10-23)
FEATURE_BIN rustfmt 1.7.0-nightly (eeac8b9 2023-10-28)

[calebcartwright]

👍