env.sh: do not set CC=clang in nix(os)

That seems to be broken:

  external/com_google_absl/absl/base/config.h:56:10: fatal error: 'cstddef' file not found

We should probably make it work some day, but let's unbreak things
first.

Change-Id: I5bc6892f8c6353a626f5ca103805c9a0e9a0d494
Reviewed-on: https://gerrit.hackerspace.pl/c/hscloud/+/1726
Reviewed-by: informatic <informatic@hackerspace.pl>
diff --git a/env.sh b/env.sh
index 578bef1..d9259f3 100644
--- a/env.sh
+++ b/env.sh
@@ -16,7 +16,7 @@
 unset -f hscloud_root
 unset -f hscloud_path
 
-if command -v clang &> /dev/null; then
+if command -v clang &> /dev/null && [ -z $HSCLOUD_NIX ]; then
     # if we have clang, use it
     # avoids weird PATH issues with linkers, eg https://github.com/bazelbuild/bazel/issues/18834
     export CC=clang