Use $HOME/.tmp as temp dir if TMPDIR is unset

[direc85]

When Rust is determining the temp directory to use during compilation, tempfile uses env::temp_dir() which in *nix returns /tmp if TMPDIR is unset. This leads to a situation where the linker can't find symbols.o file during the linking phase.

This could be mitigated already by setting TMPDIR in a .spec file, but this needs to be done in every project separately. Instead, make the compiler default to $HOME/.tmp if TMPDIR is unset instead. The patch uses $HOME, which exists in both Sailfish SDK and Platform SDK.

Patching how env::temp_dir() works in std feels overkill and could lead to other issues. Hence this patch targets the compiler itself during compiling and linking.

With Rust packages compiled with this patch, Whisperfish was able to compile without errors without the TMPDIR commands in the .spec file.

[mlehtima]

Appears to also work on OBS

[rubdos]

My only issue with this kind of patch is that it's a rather hacky workaround: if Rust decides to start using /tmp in other places where the same issue might appear, we'll have to keep throwing in this logic all over the place. Your call @mlehtima!

[mlehtima]

I think this is fine for now, if we figure out a better way or if there is need to rethink this in the future if rust changes we can always change it later.

[rubdos]

@mlehtima would you like to pull it in via the Sailfish repo, or do I have you take the scraps and put everything together? :-)