Issue description

This is the continuation for the problem in #55998 (comment).
cc @veprbl

The problem is that clang and gcc on macOS doesn't add executable rights if the output file was already created without executable rights.

Steps to reproduce

cd /tmp
touch executableTest
echo "int main() {}" > executableTest.c
nix-shell -p clang --pure --run "clang executableTest.c -o executableTest"
./executableTest

It works with gcc but still not with clang if I checkout nixpkgs master at 84be644 go into the checkout directory and do the following:

cd /tmp
touch executableTest
echo "int main() {}" > executableTest.c
nix-shell -p gcc --pure --run "gcc executableTest.c -o executableTest"
./executableTest

Technical details

Please run nix-shell -p nix-info --run "nix-info -m" and paste the
results.

• system: "x86_64-darwin"
• host os: Darwin 18.2.0, macOS 10.14.3
• multi-user?: no
• sandbox: no
• version: nix-env (Nix) 2.1.3
• channels(thomad): "nixpkgs-19.03pre171497.cc1fa81489e"
• nixpkgs: /Users/thomad/.nix-defexpr/channels/nixpkgs