Use positive source filtering for the standalone functional tests job

Additionally this skipping of the building is reimplemented to be a bit
more robust and use the same idioms as the functionality for skipping
the tests. In particular, it will now work even if the source files
exist, so we can do this during development too.

flake.nix

@@ -59,29 +59,40 @@
         # that would interfere with repo semantics.
         fileset.fileFilter (f: f.name != ".gitignore") ./.;
 
+      configureFiles = fileset.unions [
+        ./.version
+        ./configure.ac
+        ./m4
+        # TODO: do we really need README.md? It doesn't seem used in the build.
+        ./README.md
+      ];
+
+      topLevelBuildFiles = fileset.unions [
+        ./local.mk
+        ./Makefile
+        ./Makefile.config.in
+        ./mk
+      ];
+
+      functionalTestFiles = fileset.unions [
+        ./tests/functional
+        (fileset.fileFilter (f: lib.strings.hasPrefix "nix-profile" f.name) ./scripts)
+      ];
+
       nixSrc = fileset.toSource {
         root = ./.;
         fileset = fileset.intersect baseFiles (fileset.unions [
-          ./.version
+          configureFiles
+          topLevelBuildFiles
           ./boehmgc-coroutine-sp-fallback.diff
-          ./bootstrap.sh
-          ./configure.ac
           ./doc
-          ./local.mk
-          ./m4
-          ./Makefile
-          ./Makefile.config.in
           ./misc
-          ./mk
           ./precompiled-headers.h
           ./src
-          ./tests/functional
           ./unit-test-data
           ./COPYING
           ./scripts/local.mk
-          (fileset.fileFilter (f: lib.strings.hasPrefix "nix-profile" f.name) ./scripts)
-          # TODO: do we really need README.md? It doesn't seem used in the build.
-          ./README.md
+          functionalTestFiles
         ]);
       };
 
@@ -252,7 +263,6 @@
       testNixVersions = pkgs: client: daemon: with commonDeps { inherit pkgs; }; with pkgs.lib; pkgs.stdenv.mkDerivation {
         NIX_DAEMON_PACKAGE = daemon;
         NIX_CLIENT_PACKAGE = client;
-        HAVE_LOCAL_NIX_BUILD = false;
         name =
           "nix-tests"
           + optionalString
@@ -261,7 +271,14 @@
             "-${client.version}-against-${daemon.version}";
         inherit version;
 
-        src = nixSrc;
+        src = fileset.toSource {
+          root = ./.;
+          fileset = fileset.intersect baseFiles (fileset.unions [
+            configureFiles
+            topLevelBuildFiles
+            functionalTestFiles
+          ]);
+        };
 
         VERSION_SUFFIX = versionSuffix;
 
@@ -271,19 +288,20 @@
 
         enableParallelBuilding = true;
 
-        configureFlags = testConfigureFlags; # otherwise configure fails
+        configureFlags =
+          testConfigureFlags # otherwise configure fails
+          ++ [ "--disable-build" ];
+        dontBuild = true;
         doInstallCheck = true;
 
-        buildPhase = ''
-          # Remove the source files to make sure that we're not accidentally rebuilding Nix
-          rm src/**/*.cc
-        '';
-
         installPhase = ''
           mkdir -p $out
         '';
 
-        installCheckPhase = "make installcheck -j$NIX_BUILD_CORES -l$NIX_BUILD_CORES";
+        installCheckPhase = ''
+          mkdir -p src/nix-channel
+          make installcheck -j$NIX_BUILD_CORES -l$NIX_BUILD_CORES
+        '';
       };
 
       binaryTarball = nix: pkgs: