util/disk_cache: fix ~/.cache/ permissions
XDG Base Dir spec says:
If, when attempting to write a file, the destination directory is non-existent an attempt should be made to create it with permission
0700
. If the destination directory exists already the permissions should not be changed.
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/4103
Fixes: 87ab26b2 ("glsl: Add initial functions to implement an on-disk cache")
Edited by Eric Engestrom