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.
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