libostree/sepolicy: get rid of a g_setenv() call

libostree/sepolicy: get rid of a g_setenv() call

This removes a 'g_setenv()' call, which could potentially be unsafe
in a multi-thread context.
The current libselinux codebase does not seem to check for
`LIBSELINUX_DISABLE_PCRE_PRECOMPILED`, so I think this has no effects
nowadays.
Additionally, I could not find any reference to it in libselinux
git history, so I'm not sure if it ever played any role at all.

My current understanding is that this is coming from version
incompatibilities between an older libselinux in the build environment
and a newer policy (with precompiled regexs) in the target.
But from the ML discussion I found, I think it eventually got
solved in a different way, possibly by avoiding the policy binary
caches.

Refs:
 * https://www.spinics.net/lists/selinux/msg14822.html
 * https://github.com/ostreedev/ostree/pull/2513#discussion_r781042884