lock in macOS version for Fleet Desktop workers (#16009)

Implementing a safety measure to prevent issues like #15910 in
production.

Setting the macOS version explicitly avoids unexpected changes in the
builder runtime, ensuring the Fleet Desktop executable remains
compatible.

As of this commit, 'macos-latest' refers to 'macos-12'. We're aligning
the worker to this version, although building on macOS 13.x (presently
in GitHub workers' beta) should also be viable.

.github/workflows/fleet-and-orbit.yml

@@ -203,7 +203,13 @@ jobs:
     strategy:
       matrix:
         go-version: ['${{ vars.GO_VERSION }}']
-    runs-on: macos-latest
+    # Set macOS version to '12' (current equivalent to macos-latest) for
+    # building the binary. This ensures compatibility with macOS version 13 and
+    # later, avoiding runtime errors on systems using macOS 13 or newer.
+    #
+    # Note: Update this version to '13' once GitHub marks macOS 13 as stable
+    # or if we revise our minimum supported macOS version.
+    runs-on: macos-12
     steps:
 
     - name: Install Go

.github/workflows/generate-desktop-targets.yml

@@ -31,7 +31,13 @@ permissions:
 
 jobs:
   desktop-macos:
-    runs-on: macos-latest
+    # Set macOS version to '12' (current equivalent to macos-latest) for
+    # building the binary. This ensures compatibility with macOS version 13 and
+    # later, avoiding runtime errors on systems using macOS 13 or newer.
+    #
+    # Note: Update this version to '13' once GitHub marks macOS 13 as stable
+    # or if we revise our minimum supported macOS version.
+    runs-on: macos-12
     steps:
 
       - name: Install Go