Skip beta builds on macos an windows

While we should keep testing MacOS and Windows builds (especially
Winblowz since its somehow still cooperative multitasking within a
process in 2025), there's not a lot of need to test rustc beta
builds on them, which we mostly do just to ensure we aren't going
to be broken by a future rustc update that breaks some API (which
is exceedingly rare).

This should get our CI queueing times back close to zero on all
jobs.

Author: TheBlueMatt

.github/workflows/build.yml

@@ -35,6 +35,10 @@ jobs:
         exclude:
           - platform: windows-latest
             toolchain: 1.63.0
+          - platform: windows-latest
+            toolchain: beta
+          - platform: macos-latest
+            toolchain: beta
     runs-on: ${{ matrix.platform }}
     steps:
       - name: Checkout source code