test: readd TMPDIR to LD_LIBRARY_PATH for run-make

It was accidentally removed in #15006 and that somehow got past the
build bots, causing `src/test/run-make/c-dynamic-dylib` to fail on at
least my linux system.

This resolves #15103 (thanks to @alexcrichton!).
auto merge of #15114 : ben0x539/rust/run-make-libpath, r=alexcrichton

It was accidentally removed in #15006 and that somehow got past the
build bots, causing `src/test/run-make/c-dynamic-dylib` to fail on at
least my linux system.

This resolves #15103 (thanks to @alexcrichton!).