ci/cd: fetch x264 from the GitHub mirror (overlay port)

vcpkg's x264 port downloads from code.videolan.org, whose GitLab serves
the archive behind an anti-bot challenge: CI runners hit curl 7/35
(connection/SSL) or 404 at random, so whether a vcpkg install succeeded
was a coin flip. This overlay port is a verbatim copy of the builtin
port at baseline 771b0a2e with vcpkg_from_gitlab swapped for
vcpkg_from_github against mirror/x264 (byte-identical archive, same
SHA512); both workflows now pass --overlay-ports tooling/vcpkg-ports.

Verified locally in debian:12: the port downloaded from
github.com/mirror/x264 and built libx264.a. No other port in the tree
uses code.videolan.org (dav1d/x265 already fetch from GitHub).

.github/workflows/ci.yml

@@ -315,7 +315,7 @@ jobs:
           # error — so retry here. vcpkg resumes from its archive and
           # download caches, so a repeat attempt is cheap.
           for attempt in 1 2 3; do
-            vcpkg install --triplet ${{ matrix.triplet }} --overlay-triplets tooling/vcpkg-triplets/release && exit 0
+            vcpkg install --triplet ${{ matrix.triplet }} --overlay-triplets tooling/vcpkg-triplets/release --overlay-ports tooling/vcpkg-ports && exit 0
             echo "vcpkg install failed (attempt $attempt); retrying"
             sleep 15
           done
@@ -326,7 +326,7 @@ jobs:
         shell: pwsh
         run: |
           for ($i = 1; $i -le 3; $i++) {
-            vcpkg install --triplet ${{ matrix.triplet }} --overlay-triplets tooling/vcpkg-triplets/release
+            vcpkg install --triplet ${{ matrix.triplet }} --overlay-triplets tooling/vcpkg-triplets/release --overlay-ports tooling/vcpkg-ports
             if ($LASTEXITCODE -eq 0) { exit 0 }
             Write-Host "vcpkg install failed (attempt $i); retrying"
             Start-Sleep -Seconds 15