sagemathgh-36941: Reduce number of systems test with minimal config

    
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Alternative solution for sagemath#36726. Since the purpose of the "minimal" ci
runs is to check the documentation of the minimal build requirements, it
is overkill to run it for all systems. Thus, we only run it for "ubuntu-
focal", which is also the default system for PRs.

This also hugely reduces the load of onto the ci, somewhat leviating the
issues introduced by sagemath#36616 (see eg
sagemath#36616 (comment)).

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### ⌛ Dependencies

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URL: sagemath#36941
Reported by: Tobias Diez
Reviewer(s): Dima Pasechnik

.github/workflows/ci-linux.yml

@@ -146,6 +146,8 @@ jobs:
       docker_targets: "with-system-packages configured with-targets-pre"
       # FIXME: duplicated from env.TARGETS
       targets_pre: all-sage-local
+      tox_system_factors: >-
+        ["ubuntu-focal"]
       tox_packages_factors: >-
           ["minimal"]
       docker_push_repository: ghcr.io/${{ github.repository }}/
@@ -167,6 +169,8 @@ jobs:
       # FIXME: duplicated from env.TARGETS
       targets: build doc-html
       targets_optional: ptest
+      tox_system_factors: >-
+        ["ubuntu-focal"]
       tox_packages_factors: >-
           ["minimal"]
       docker_push_repository: ghcr.io/${{ github.repository }}/