CI: rework build workflow to have split target and subtarget directly

Instead of referring to a redundant job and ENV variables, rework build
workflow to accept and require split target and subtarget and use them
directly from inputs.

Rework each user and pass a JSON of tuple to matrix include with each
target/subtarget combination to test. Special notice this doesn't use
the github actions matrix combination feature but reference each
specific tuple of target and subtarget to test.

Just a cleanup no behaviour change intended.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

.github/workflows/packages.yml

@@ -37,11 +37,14 @@ jobs:
       fail-fast: False
       matrix:
         include:
-          - target: malta/be
-          - target: x86/64
+          - target: malta
+            subtarget: be
+          - target: x86
+            subtarget: 64
     uses: ./.github/workflows/build.yml
     with:
       target: ${{ matrix.target }}
+      subtarget: ${{ matrix.subtarget }}
       build_kernel: true
       build_all_kmods: true
       build_all_modules: true