Handle stdout from shell hooks

Before this commit, any `shellHook` which emitted to stdout would cause
the action to fail with a message like `<valid name of a variable from
the shell being loaded>: Invalid variable name`, as the hook's output
would get interpreted as part of the output of `env -0` and `echo
$PATH`.

After this commit, those commands are prefixed by a random delimiter,
and all output before that delimiter is ignored.

Type: fix
Issue: 11

flake.nix

@@ -16,7 +16,7 @@
     packages = eachSystem ({pkgs, ...}: {
       default = pkgs.writeShellApplication {
         name = "nix-develop-gha";
-        runtimeInputs = [pkgs.gnugrep pkgs.openssl.bin pkgs.coreutils];
+        runtimeInputs = [pkgs.gnugrep pkgs.openssl.bin pkgs.coreutils pkgs.gnused];
         text = builtins.readFile ./nix-develop-gha.sh;
       };
     });

nix-develop-gha.sh

@@ -6,7 +6,8 @@ set -euo pipefail
 IFS=" " read -r -a arguments <<<"${@:-./#default}"
 
 with_nix_develop() {
-	nix develop "${arguments[@]}" --command "$@"
+	delimiter=$(openssl rand -base64 18)
+	nix develop "${arguments[@]}" --command bash -c "echo $delimiter; $*" | sed -n "\|$delimiter|,\$p" | tail -n +2
 }
 
 with_nix_develop true # Exit immediately if build fails
@@ -35,10 +36,10 @@ while IFS='=' read -r -d '' n v; do
 		continue
 	fi
 	printf "%s=%s\n" "$n" "$v" >>"${GITHUB_ENV:-/dev/stderr}"
-done < <(with_nix_develop env -0)
+done < <(with_nix_develop "env -0")
 
 # Read the nix environment's $PATH into an array
-IFS=":" read -r -a nix_path_array <<<"$(with_nix_develop bash -c "echo \$PATH")"
+IFS=":" read -r -a nix_path_array <<<"$(with_nix_develop "echo \$PATH")"
 
 # Iterate over the PATH array in reverse
 #