[CI] Cover the legacy distros and the slow macOS runner in a nightly

Every pull request built ten Linux legs and five macOS/Windows legs, including
the previous release of each distro and macOS 13 on the Intel runner. Those
legs exist to catch a regression for users on an older distribution, which a
change being reviewed almost never does, and macOS 13 Intel is the slowest leg
of the matrix - 28 minutes against 19 for the rest.

They now run in a nightly build of the default branch and on tag builds, which
is when a change actually ships. Pull requests keep the current release of each
distro, the sanitizer leg and the coverage leg.

.github/workflows/desktop-build.yml

@@ -33,6 +33,11 @@ on:
       - 'CMakeLists.txt'
       - 'vcpkg.json'
       - 'vcpkg'    # needed to match submodule bumps (gitlink)
+  # The legs the pull-request matrix leaves out run here instead, so the legacy distros and
+  # the slow macOS runner stay covered without gating a pull request on them. Tags still
+  # build every leg, because a tag is what ships.
+  schedule:
+    - cron: '23 3 * * *'
 
 # Cancel earlier, unfinished runs of this workflow on the same branch (unless on tag --> release)
 concurrency:
@@ -152,6 +157,7 @@ jobs:
 
           - distro: Debian
             version: 12
+            nightly: yes    # previous stable, still covered by the nightly
 
             package: DEB
             test: skip    # Running tests on all distros is superfluous
@@ -163,6 +169,7 @@ jobs:
 
           - distro: Fedora
             version: 43
+            nightly: yes    # previous release, still covered by the nightly
 
             package: RPM
             test: skip    # Running tests on all distros is superfluous
@@ -179,6 +186,7 @@ jobs:
 
           - distro: Ubuntu
             version: 24.04
+            nightly: yes    # previous LTS, still covered by the nightly
 
             package: DEB
             test: skip    # Running tests on all distros is superfluous
@@ -219,6 +227,9 @@ jobs:
 
     name: ${{ matrix.distro }} ${{ matrix.version }}${{ matrix.variant && format(' {0}', matrix.variant) || '' }}
     needs: configure
+    # A leg marked nightly runs in the nightly and on tags, never on a pull request or a
+    # merge, so nothing that reviews a change waits for a distro nobody is changing for.
+    if: matrix.nightly != 'yes' || github.event_name == 'schedule' || github.ref_type == 'tag'
     runs-on: ubuntu-latest    # https://github.com/actions/runner-images
     continue-on-error: ${{ matrix.allow-failure == 'yes' }}
     timeout-minutes: 70
@@ -453,6 +464,7 @@ jobs:
             target: 13    # EOL 2025-09-15
             runner: macos-15-intel    # https://github.com/actions/runner-images/blob/main/images/macos/macos-15-Readme.md
                                       # macos-26-intel is very slow and fails in CPack during DMG config if not increasing Finder timeout
+            nightly: yes    # EOL target on the slowest runner: nightly and tags only
 
             ccache_eviction_age: 7d
             cmake_generator: Ninja
@@ -543,6 +555,7 @@ jobs:
 
     name: ${{ matrix.os }} ${{ matrix.target }}${{ matrix.soc == 'Intel' && ' Intel' || '' }}${{ matrix.type == 'Debug' && ' Debug' || '' }}
     needs: [configure, vcpkg-baseline]
+    if: matrix.nightly != 'yes' || github.event_name == 'schedule' || github.ref_type == 'tag'
     runs-on: ${{ matrix.runner }}
     timeout-minutes: 100
     env: