fix: Only use /run/user/<UID>/ccache-tmp if writable

fix: Only use /run/user/<UID>/ccache-tmp if writable

The fix for #984 addressed a problem when /run/user/0 already exists and
ccache is run with fakeroot. However, it didn’t handle the case when
/run/user/0/ccache-tmp already exists, which will happen for instance if
the real root user has run ccache at least once.

Fix this by using access(2) to verify that the ccache-tmp directory is
writable. Note: Can’t just check the mode bits of the directory since
they appear OK since fakeroot fakes the UID.

Fixes #1044.