nix-shell: stop using dynamic format strings!!

This was always a terrible idea independently of whether it crashes.
Stop doing it!

This commit was verified by running nix-shell on a trivial derivation
with --debug --verbose to get the vomit-level output of the shell rc
file and then diffing it before/after this change. I have reasonable
confidence it did not regress anything, though this code is genuinely
really hard to follow (which is a second reason that I split it into two
fmt calls).

Fixes: https://git.lix.systems/lix-project/lix/issues/533
Change-Id: I8e11ddbece2b12749fda13efe0b587a71b00bfe5

tests/functional/meson.build

@@ -115,6 +115,7 @@ functional_tests_scripts = [
   'check.sh',
   'nix-shell/basic.sh',
   'nix-shell/structured-attrs.sh',
+  'nix-shell/regression-533.sh',
   'check-refs.sh',
   'build-remote-input-addressed.sh',
   'secure-drv-outputs.sh',

tests/functional/nix-shell/regression-533.sh

@@ -0,0 +1,18 @@
+source ../common.sh
+
+clearStore
+
+evil=$(cat <<-'EOF'
+builtins.derivation {
+  name = "evil-kbity";
+  system = "x86_64-darwin";
+  builder = "/bin/sh";
+  args = [ "-c" "> $out" ];
+  __structuredAttrs = true;
+  env.oops = "lol %s";
+}
+EOF
+)
+
+# This should not crash
+nix-shell --expr "$evil" --run 'echo yay' | grepQuiet yay