CI: environment.sh: set -eu

the script is sourced so errexit+nounset is perhaps not yet set

But it is better to exit early, especially when setting the environment
variables that are considered a prerequisite.

Author: MartinPulec

.github/scripts/environment.sh

@@ -9,6 +9,8 @@
 ## - **apple_key_p12_b64** - [mac only] base64-encoded $KEY_FILE (using
 ##                           password $KEY_FILE_PASS)
 
+set -eu
+
 if expr "$GITHUB_REF" : 'refs/tags/' >/dev/null; then
   TAG=${GITHUB_REF#refs/tags/}
   VERSION=${TAG#v}