ci: retry vcpkg manifest install on transient source-download failures

The first WarpCache-cold run failed because x264's source tarball on
code.videolan.org refused connections (curl error 7) and vcpkg refuses
to retry that class of error. Retry the install up to three times on
every platform; vcpkg resumes from its archive/download caches.

.github/workflows/ci.yml

@@ -89,7 +89,18 @@ jobs:
             vcpkg-${{ runner.os }}-
 
       - name: Install dependencies (vcpkg manifest)
-        run: vcpkg install --triplet x64-linux
+        run: |
+          # Source tarballs come from third-party hosts (x264 lives on
+          # code.videolan.org); a transient connection failure aborts the
+          # whole install — vcpkg refuses to retry that class of curl
+          # 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 x64-linux && exit 0
+            echo "vcpkg install failed (attempt $attempt); retrying"
+            sleep 15
+          done
+          exit 1
 
       # Explicit restore/save pair: the old `save-always: true` on the
       # combined step does not actually save on a failed job (the action
@@ -328,7 +339,16 @@ jobs:
             vcpkg-${{ runner.os }}-
 
       - name: Install dependencies (vcpkg manifest)
-        run: vcpkg install --triplet x64-windows
+        run: |
+          # See the Linux job: transient source-download failures abort
+          # the manifest install; retry before giving up.
+          for ($i = 1; $i -le 3; $i++) {
+            vcpkg install --triplet x64-windows
+            if ($LASTEXITCODE -eq 0) { exit 0 }
+            Write-Host "vcpkg install failed (attempt $i); retrying"
+            Start-Sleep -Seconds 15
+          }
+          exit 1
 
       # Explicit restore/save pair (see the Linux job: `save-always` did
       # not save after a failed job, so Windows rebuilt FFmpeg from source
@@ -450,7 +470,15 @@ jobs:
             vcpkg-${{ runner.os }}-
 
       - name: Install dependencies (vcpkg manifest)
-        run: vcpkg install --triplet arm64-osx
+        run: |
+          # See the Linux job: transient source-download failures abort
+          # the manifest install; retry before giving up.
+          for attempt in 1 2 3; do
+            vcpkg install --triplet arm64-osx && exit 0
+            echo "vcpkg install failed (attempt $attempt); retrying"
+            sleep 15
+          done
+          exit 1
 
       - name: Save vcpkg artifacts
         if: always()
@@ -647,7 +675,15 @@ jobs:
             vcpkg-${{ runner.os }}-${{ matrix.arch }}-openkylin-
 
       - name: Install dependencies (vcpkg manifest)
-        run: vcpkg install --triplet ${{ matrix.triplet }}
+        run: |
+          # See the Linux job: transient source-download failures abort
+          # the manifest install; retry before giving up.
+          for attempt in 1 2 3; do
+            vcpkg install --triplet ${{ matrix.triplet }} && exit 0
+            echo "vcpkg install failed (attempt $attempt); retrying"
+            sleep 15
+          done
+          exit 1
 
       - name: Save vcpkg artifacts
         if: always()