ci(garnix): explicitly declare systems

as we now create outputs for each system in `lib.systems.flakeExposed`,
we should ensure garnix only builds the systems we officially support

dev shells are also no longer built as they really don't need to be
"built" on the developer's machine in any case; everything should be
cached

Signed-off-by: seth <getchoo@tuta.io>

garnix.yaml

@@ -1,7 +1,10 @@
 builds:
   exclude:
+    # Currently broken on Garnix's end
     - "*.x86_64-darwin.*"
   include:
     - "checks.x86_64-linux.*"
-    - "devShells.*.*"
-    - "packages.*.*"
+    - "packages.x86_64-linux.*"
+    - "packages.aarch64-linux.*"
+    - "packages.x86_64-darwin.*"
+    - "packages.aarch64-darwin.*"