tests.makeBinaryWrapper: Prevent rebuilds when Nix files change

The source included way more files than it really needed.
This commit limits it to exactly those it needs.

This also makes sure that no rebuild is necessary when any Nix
file changes, in particular useful when we reformat the Nix files.

pkgs/test/make-binary-wrapper/default.nix

@@ -15,16 +15,25 @@ let
   makeGoldenTest = testname: runCommand "make-binary-wrapper-test-${testname}" env ''
     mkdir -p tmp/foo # for the chdir test
 
-    params=$(<"${./.}/${testname}.cmdline")
+    source=${lib.fileset.toSource {
+      root = ./.;
+      fileset = lib.fileset.unions [
+        (./. + "/${testname}.cmdline")
+        (./. + "/${testname}.c")
+        (lib.fileset.maybeMissing (./. + "/${testname}.env"))
+      ];
+    }}
+
+    params=$(<"$source/${testname}.cmdline")
     eval "makeCWrapper /send/me/flags $params" > wrapper.c
 
-    diff wrapper.c "${./.}/${testname}.c"
+    diff wrapper.c "$source/${testname}.c"
 
-    if [ -f "${./.}/${testname}.env" ]; then
+    if [ -f "$source/${testname}.env" ]; then
       eval "makeWrapper ${envCheck} wrapped $params"
       env -i ./wrapped > env.txt
       sed "s#SUBST_ARGV0#${envCheck}#;s#SUBST_CWD#$PWD#" \
-        "${./.}/${testname}.env" > golden-env.txt
+        "$source/${testname}.env" > golden-env.txt
       if ! diff env.txt golden-env.txt; then
         echo "env/argv should be:"
         cat golden-env.txt