build: replace archlinux by openbsd

This commit drops Arch Linux in favor of a BSD so there's more variety
in CI.

Link: https://lists.sr.ht/~rjarry/aerc-devel/%3C20220122033806.91728-1-ktprograms%40gmail.com%3E

.builds/archlinux.yml

@@ -1,13 +0,0 @@
-image: archlinux
-packages:
-- go
-- scdoc
-sources:
-- https://git.sr.ht/~rjarry/aerc
-tasks:
-- build: |
-    cd aerc
-    make
-- test: |
-    cd aerc
-    go test ./...

.builds/openbsd.yml

@@ -0,0 +1,14 @@
+---
+image: openbsd/latest
+packages:
+  - go
+  - scdoc
+sources:
+  - https://git.sr.ht/~rjarry/aerc
+tasks:
+  - build: |
+        cd aerc
+        make
+  - test: |
+        cd aerc
+        go test ./...