[build] Overhaul Linux builds and refactor release workflow (#13997)

- Use `manylinux-shared` images for Linux builds
- Discontinue `yt-dlp_linux_armv7l`/`linux_armv7l_exe` release binary
- Add `yt-dlp_linux_armv7l.zip`/`linux_armv7l_dir` release binary
- Add `yt-dlp_musllinux` and `yt-dlp_musllinux_aarch64` release binaries
- Migrate `linux_exe` build strategy from staticx+musl to manylinux2014/glibc2.17
- Rewrite release.yml's "unholy bash monstrosity" as devscripts/setup_variables.py

Closes #10072, Closes #10630, Closes #10578, Closes #13976, Closes #13977, Closes #14106
Authored by: bashonly

.github/workflows/cache-warmer.yml

@@ -0,0 +1,22 @@
+name: Keep cache warm
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: '0 22 1,6,11,16,21,27 * *'
+
+jobs:
+  build:
+    if: |
+      vars.KEEP_CACHE_WARM || github.event_name == 'workflow_dispatch'
+    uses: ./.github/workflows/build.yml
+    with:
+      version: '999999'
+      channel: stable
+      unix: false
+      linux: false
+      linux_armv7l: true
+      musllinux: false
+      macos: true
+      windows: true
+    permissions:
+      contents: read