nixos-rebuild: inline get-version-suffix script

It has no other consumers anyway. Also saves some work.

nixos/modules/installer/tools/get-version-suffix

@@ -1,23 +0,0 @@
-getVersion() {
-    local dir="$1"
-    rev=
-    gitDir="$dir/.git"
-    if [ -e "$gitDir" ]; then
-        if [ -z "$(type -P git)" ]; then
-            echo "warning: Git not found; cannot figure out revision of $dir" >&2
-            return
-        fi
-        cd "$dir"
-        rev=$(git --git-dir="$gitDir" rev-parse --short HEAD)
-        if git --git-dir="$gitDir" describe --always --dirty | grep -q dirty; then
-            rev+=M
-        fi
-    fi
-}
-
-if nixpkgs=$(nix-instantiate --find-file nixpkgs "$@"); then
-    getVersion $nixpkgs
-    if [ -n "$rev" ]; then
-        echo ".git.$rev"
-    fi
-fi