Allow overriding temporary directory with $TMPDIR env var

[catkins]

Currently, the images and SOCI artifacts are stored in /tmp during processing, which is a very sensible default.

We run this in our CI environment where we have a 1GiB tmpfs volume at /tmp, so on some images we get this in our build logs.

{"level":"error","error":"errors encountered while building soci layers; write /tmp/tmp.3943925700: no space left on device; write /tmp/tmp.1788043098: no space left on device","RegistryURL":"<accountid>.dkr.ecr.<region>.amazonaws.com","ImageDigest":"<digest>","time":"2024-09-30T03:22:06Z","message":"SOCI index build error"}

I'd love to be able to override the temporary directory used by this program using the built-in $TMPDIR env var recognised by the Go stdlib, for some specific builds.

References

https://pkg.go.dev/os#MkdirTemp

If dir is the empty string, MkdirTemp uses the default directory for temporary files, as returned by TempDir.

https://pkg.go.dev/os#TempDir

On Unix systems, it returns $TMPDIR if non-empty, else /tmp.

[kichik]

Thanks!