[NO TESTS NEEDED] Fix #11418 - Default TMPDIR to /tmp on OS X

[MichaelAnckaert]

This PR fixes bug #11418 - on OS X there was a bug where podman would fail when the TMPDIR environment variable was empty.

This commit fixes that issue by defaulting to /tmp when that environment variable is empty.

[mheon]

pkg/machine/qemu/options_darwin.go:6:2: imported and not used: "github.com/pkg/errors"

Can you remove the github.com/pkg/errors import from that file to fix CI? Also, I've added [NO TESTS NEEDED] to your PR title to fix CI complaining that you did not add a test.

[maxandersen]

should it not be like ~/tmp as /tmp my not be user writable ?

[MichaelAnckaert]

@maxandersen The /tmp directory is also used on OS X and my research seems to indicate that this is always present in all cases.

I updated the changes to remove the unused import.

[rhatdan]

/approve
LGTM
@baude @ashley-cui @jwhonce PTAL

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

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[jwhonce]

/lgtm