Coverity CI: set -u

If the curl for filename fails, do not succeed with restoring the cache
with partial match. This will return false which may be unexpected
(typically this is correlated when the Coverity servers are down).

Author: MartinPulec

.github/workflows/coverity-scan.yml

@@ -25,6 +25,7 @@ jobs:
     - name: Get Coverity tool name # the file name contains version and is used as the cache key
       id: tool
       run: |
+        set -u
         FILENAME=$(curl -LIf "https://scan.coverity.com/download/linux64\
         ?token=$coverity_token&project=UltraGrid" |
         sed -n '/content-disposition/s/.*\"\(.*\)\"/\1/p')