Update branch to main from master in snyk workflow

The default branch of this repository has been updated to main, but the
branch name was not updated in this workflow. This means the workflow
hasn’t been running when commits have been merged to main (e.g. if you
look at the builds on [the last commit to main on
github](bb1b98e),
there’s no run of the snyk workflow).

As such, our dependency vulnerability notifications haven’t been up to
date! This commit updates the branch name so that the notifications will
be up to date.

.github/workflows/snyk.yml

@@ -3,7 +3,7 @@ name: Snyk
 on:
   push:
     branches:
-      - master
+      - main
   workflow_dispatch:
 
 jobs: