Set SKIP_CC_CVMFS=1 in nix-shell pure mode to avoid CVMFS pollution

pkgs/nix/cvmfs.patch

@@ -0,0 +1,12 @@
+diff --git a/scripts/nix-build.in b/scripts/nix-build.in
+index bb61e1c..d58d687 100755
+--- a/scripts/nix-build.in
++++ b/scripts/nix-build.in
+@@ -276,6 +276,7 @@ foreach my $expr (@exprs) {
+             }
+             # NixOS hack: prevent /etc/bashrc from sourcing /etc/profile.
+             $ENV{'__ETC_PROFILE_SOURCED'} = 1;
++            $ENV{'SKIP_CC_CVMFS'} = 1;
+         }
+         $ENV{'NIX_BUILD_TOP'} = $ENV{'TMPDIR'} = $ENV{'TEMPDIR'} = $ENV{'TMP'} = $ENV{'TEMP'} = $tmp;
+         $ENV{'NIX_STORE'} = $Nix::Config::storeDir;