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

@@ -34,7 +34,9 @@ jobs:
         toolchain: [ stable, beta, 1.63.0 ] # 1.63.0 is the MSRV for all crates but `lightning-transaction-sync`.
         exclude:
           - platform: windows-latest
-            toolchain: 1.63.0
+            toolchain: [ 1.63.0, beta ]
+          - platform: macos-latest
+            toolchain: [ beta ]
     runs-on: ${{ matrix.platform }}
     steps:
       - name: Checkout source code