runtime: Cope with /tmp being a symlink
If /tmp is a symlink in the current execution environment, then when we ask bwrap to bind-mount /tmp/pressure-vessel-wrap.XXXXXX/overrides over itself, it will try to create the parent directory /tmp, and fail because there's a symlink in the way. Canonicalize the path and use that instead.
Partially addresses https://github.com/ValveSoftware/steam-runtime/issues/321 (symlinks "above" the home directory have the same problem, but are harder to fix).
/cc @denittis