Update build.yml to fix BSD builds (#456)

Author: perkelix

.github/workflows/build.yml

@@ -29,7 +29,7 @@ jobs:
       run: make tests
 
   openbsd:
-    runs-on: macos-12
+    runs-on: ubuntu-latest
     steps:
     - name: Bootstrap OpenBSD-latest
       uses: mario-campos/emulate@v1
@@ -56,7 +56,7 @@ jobs:
         make tests
 
   freebsd:
-    runs-on: macos-12
+    runs-on: ubuntu-latest
     steps:
     - name: Bootstrap FreeBSD-latest
       uses: mario-campos/emulate@v1
@@ -82,7 +82,7 @@ jobs:
          make tests
 
   netbsd:
-    runs-on: macos-12
+    runs-on: ubuntu-latest
     steps:
     - name: Bootstrap NetBSD-latest
       uses: mario-campos/emulate@v1