Motivation for this change

While implementing #64319 it was discovered that it is not possible to override src for llvm and clang because the string interpolation of src is used to define an unpackPhase. This PR changes it to using environment variable so that it can be easily changed with overrideAttrs.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested compilation of all pkgs that depend on this change using nix-review pr --build-args "--option build-use-sandbox false" 64647 -p clang_35 -p clang_38 -p clang_39 -p clang_4 -p clang_5 -p clang_6 -p clang_7 -p clang_8 -p llvm_35 -p llvm_38 -p llvm_39 -p llvm_4 -p llvm_5 -p llvm_6 -p llvm_7 -p llvm_8
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.