GitHub Actions: first CI/CD build for Mac based on GitHub Actions

This feature is in beta right now and might change without notice, but instead
of dealing with the broken Travis Mac builds, this does seem progress.

The build artifact seems to work, but it's a bit more painful to get to. Go to
https://github.com/Subsurface-divelog/subsurface/actions and click on the
corresponding run - it's then in the top right corner under Artifacts. The one
oddity is that after unzipping the file you need to manually make
Contents/MacOS/Subsurface executable.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

scripts/build.sh

@@ -12,6 +12,10 @@
 # in subsurface/build
 
 # create a log file of the build
+
+# don't keep going if we run into an error
+set -e
+
 exec 1> >(tee build.log) 2>&1
 
 SRC=$(pwd)