test: Attempt to fix nightly-linux

The nightly builds on linux have been failing over the past few days due to a
malformed LD_LIBRARY_PATH. It appears that the underlying cause is that one of
the tests, dep-info-custom, recursively invokes make but the RUSTC variable
passed down has the string "$LD_LIBRARY_PATH". This is intended to read the
host's original LD_LIBRARY_PATH, but it appears that the makefile is eagerly
expanding the "$L" to nothing, causing the original host's LD_LIBRARY_PATH to be
ignored.

This fix removes passing the string "$LD_LIBRARY_PATH" and rather expands it
eagerly to ensure that escaping doesn't happen at a later stage. I'm still not
entirely sure why the makefile is interpreting the dollar as a variable, but
this seems to fix the issue.