flake/ci: re-enable most tests on buildbot

Most still run on all platforms, but many now only run on x86_64-linux.

tests/default.nix

@@ -5,6 +5,7 @@
   linkFarm,
   self, # The flake instance
   system ? pkgs.stdenv.hostPlatform.system,
+  allSystems ? true,
 }:
 let
   autoArgs = pkgs // {
@@ -31,8 +32,15 @@ let
   callTests = lib.callPackagesWith autoArgs;
 
   selfPackages = self.packages.${system};
+
+  # For tests that CI should only build on one system,
+  # This is true when on that system.
+  #
+  # TODO: consider refactoring tests/default.nix so that some tests are
+  # defined by it, while others are defined elsewhere...
+  buildForThisSystem = allSystems || system == "x86_64-linux";
 in
-{
+lib.optionalAttrs buildForThisSystem {
   extra-args-tests = callTest ./extra-args.nix { };
   extend = callTest ./extend.nix { };
   extra-files = callTest ./extra-files.nix { };
@@ -47,10 +55,12 @@ in
   lsp-all-servers = callTest ./lsp-servers.nix { };
 }
 # Expose some tests from the docs as flake-checks too
-// lib.optionalAttrs (selfPackages ? docs) {
+// lib.optionalAttrs (selfPackages ? docs && buildForThisSystem) {
   # Individual tests can be run using: nix build .#docs.user-configs.tests.<test>
   docs-user-configs = linkFarm "user-configs-tests" selfPackages.docs.user-configs.tests;
 }
+
+# These are always built on all systems, even when `allSystems = false`
 // callTests ./platforms { }
 # Tests generated from ./test-sources
 # Grouped as a number of link-farms in the form { test-1, test-2, ... test-N }