ci: add SPDX headers

I've checked over each of these and they're straightforward applications
of the relevant Github Actions.

.github/workflows/freebsd.yml

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: 0BSD
+
 name: FreeBSD
 
 on:

.github/workflows/netbsd.yml

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: 0BSD
+
 name: NetBSD
 
 on:

.github/workflows/openbsd.yml

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: 0BSD
+
 name: OpenBSD
 
 on: