ci: add job to ensure static builds eval and configure

Static builds have broken and been fixed multiple times now[1][2]. It's
too expensive to add another "build Lix" job to CI, but a lot
of failures happen at eval time or during build configuration. We can at
least catch those.

[1]: https://git.lix.systems/lix-project/lix/commit/ac80a11300eb60006b7de90fb10ad6789e5beeed
[2]: https://git.lix.systems/lix-project/lix/commit/c82af241f58d7aeaecbb2a6d17bd8d7bfcc834ce

Change-Id: I5618f1ba7554324e60f4f510e70b1ccea6445f4e

flake.nix

@@ -270,6 +270,22 @@
         # Binary package for various platforms.
         build = forAllSystems (system: self.packages.${system}.nix);
 
+        # Building Lix twice in CI is expensive, but we can catch a lot of static
+        # build regressions by at least making sure it evals and configures.
+        configure-static = lib.genAttrs linux64BitSystems (
+          system:
+          self.packages.${system}.nix-static.overrideAttrs {
+            dontBuild = true;
+            installPhase = ''
+              runHook preInstall
+
+              echo "configure-static complete. exiting with success"
+              mkdir -p "$out"
+              exit 0
+            '';
+          }
+        );
+
         # Ensure support for lowdown < 1.4 doesn't regress
         build-lowdown_1_3 = forAllSystems (
           system: