create script to determine build number

What a pain. It turns out that github.run_number is counting the number of
times a specific workflow has been run - but that's different for different
workflows, so using that won't get us a single tag with all the corresponding
build artifacts.  And sadly I can't find a simple atomic way to increase a
GitHUb repo variable, so I came up with this somewhat convoluted dance, using
the the fact that a push to an existing brach that isn't a fast-forward will
fail.

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

scripts/get-or-create-build-nr.sh

@@ -0,0 +1,60 @@
+#!/bin/bash
+# this is comically complicated - why does GitHub not have a monotonic number
+# that tracks how many times any action was kicked off? Or an atomic way to update a variable?
+# So we use the fact that git itself does a great job of preventing us from overwriting an
+# existing branch and abuse that to create atomicity
+
+SHA_BRANCH="branch-for-$1"
+
+# first - make sure git is configured so we can do stuff
+git config --global user.email "ci@subsurface-divelog.org"
+git config --global user.name "Subsurface CI"
+
+# next, clone the release repo
+[ -d nightly-builds ] || git clone https://github.com/subsurface/nightly-builds
+cd nightly-builds
+
+# this is from the main branch, so this should be the PREVIOUS build number
+latest=$(<latest-subsurface-buildnumber)
+
+# now let's see if a branch for the current SHA exists
+if git checkout $SHA_BRANCH
+then
+  # one of the other workflows created a release number already
+  latest=$(<latest-subsurface-buildnumber)
+else
+  # this is almost certainly a race between the different workflow files
+  # the main branch should have held the previous release number
+  # increment by one and write as new build number into the named branch
+  latest=$((latest+1))
+  git checkout -b $SHA_BRANCH
+  echo $latest > latest-subsurface-buildnumber
+  git commit -a -m "record build number for this SHA"
+
+  # now comes the moment of truth - are we the first one?
+  # the push will succeed for exactly one of the workflows
+  if git push https://github.com/subsurface/nightly-builds $SHA_BRANCH
+  then
+    # yay - we win! now let's make sure that we remember this number for next time
+    git checkout main
+    echo $latest > latest-subsurface-buildnumber
+    git commit -a -m "record latest build number in main branch"
+    if ! git push https://github.com/subsurface/nightly-builds main
+    then
+      echo "push to main failed - we'll lose monotonic property"
+      exit -1
+    fi
+  else
+    # someone else was faster - get the number they wrote
+    git checkout main
+    git branch -D $SHA_BRANCH
+    if ! git checkout -b $SHA_BRANCH
+    then
+      echo "push to $SHA_BRANCH failed, but switching to it failed as well"
+      exit -2
+    fi
+    latest=$(<latest-subsurface-buildnumber)
+  fi
+fi
+# if we get here, we have the next build number - wow that was weird.
+echo "Build number is $latest"