ci/toolchain: don't execute workflow by itself

This workflow should only be executed when needed by another workflow.
This saves us from doing the same job twice, and reduces the total
number of jobs from 6 to 4.

Author: d4ilyrun

.github/workflows/build-toolchain.yml

@@ -1,10 +1,6 @@
 name: Build Toolchain
 
 on:
-  pull_request:
-  push:
-    branches:
-    - master
   workflow_call:
     outputs:
       cache-key-gcc: