buildFHSenv: fixup /etc permissions

when bubblewraps tries to link all the required files in etc from the
host to the fhs environment, it will re-create the /etc directory.
It will do so with `0700` permissions. This causes permissions issues
with non-root programs when they need to access configuration in the
environment /etc.

By mounting /etc as a tmpfs early, bwrap will make the directory `0755`
as expected.

pkgs/build-support/build-fhsenv-bubblewrap/default.nix

@@ -200,6 +200,7 @@ let
       # Also, the cache needs to go to both 32 and 64 bit glibcs, for games
       # of both architectures to work.
       --tmpfs ${glibc}/etc \
+      --tmpfs /etc \
       --symlink /etc/ld.so.conf ${glibc}/etc/ld.so.conf \
       --symlink /etc/ld.so.cache ${glibc}/etc/ld.so.cache \
       --ro-bind ${glibc}/etc/rpc ${glibc}/etc/rpc \