etc: Fix install script for rpath removal

This adds detection of the relevant LD_LIBRARY_PATH-like environment variable
and appropriately sets it when testing whether binaries can run or not.
Additionally, the installation prints a recommended value if one is necessary.
syntax: De-doc comment to fix nightlies

This reverts the promotion from line-comment to doc-comment in 4989a56 to fix
the compiler-docs target.

Closes #15553
auto merge of #15550 : alexcrichton/rust/install-script, r=brson

This adds detection of the relevant LD_LIBRARY_PATH-like environment variable
and appropriately sets it when testing whether binaries can run or not.
Additionally, the installation prints a recommended value if one is necessary.

Closes #15545