CI, MAINT: restrictions on pre-release CI (scipy#14166)

* pre-release builds are currently causing a series of
CI test failures on both `master` and backport PRs (like
this one: scipy#14148)

* for the specific case of `maintenance/` branch-targeted
backports, the changes suggested here aim to prevent
the pre-release checks from running on release branch PRs
on the basis that they will inevitably break in non-relevant
ways (unsupported Python versions, transient NumPy nightly
issues, etc.--I believe it may actually be latest NumPy
in this case since Python 3.7 pre-rel has similar failures
to Python 3.10)

.github/workflows/linux.yml

@@ -53,7 +53,7 @@ jobs:
 
   test_nightly:
     name: Nightly CPython
-    if: "github.repository == 'scipy/scipy' && !contains(github.event.head_commit.message, '[ci skip]') && !contains(github.event.head_commit.message, '[skip ci]') && !contains(github.event.head_commit.message, '[skip github]') && !contains(github.ref, 'maintenance/')"
+    if: "github.repository == 'scipy/scipy' && !contains(github.event.head_commit.message, '[ci skip]') && !contains(github.event.head_commit.message, '[skip ci]') && !contains(github.event.head_commit.message, '[skip github]') && !contains(github.ref, 'maintenance/') && !contains(github.base_ref, 'maintenance/')"
     runs-on: ubuntu-latest
     strategy:
       matrix:

azure-pipelines.yml

@@ -56,6 +56,7 @@ stages:
     AZURE_CI: 'true'
   jobs:
   - job: pre_release_deps_source_dist
+    condition: eq(variables['System.PullRequest.TargetBranch'], 'master')
     pool:
       vmImage: 'ubuntu-18.04'
     steps: