Allow custom shellcheck severity

Normal severity is normal.

CI severity is set to warning, but this should be
changed to style (default) once most
of the scripts are iether gone or modernized.

.github/workflows/test.yml

@@ -44,7 +44,7 @@ jobs:
             shfmt_{tag}_linux_amd64 \
             /usr/bin/shfmt
 
-      - run: make shell-check
+      - run: SEVERITY=warning make shell-check
 
   test:
     strategy:

Makefile

@@ -13,4 +13,4 @@ shell-format:
 
 shell-check:
 	shfmt -f . | xargs shfmt -i 2 -ci -sr -kp -d
-	shfmt -f . | xargs shellcheck --color=always || true
+	shfmt -f . | xargs shellcheck --color=always --severity=$${SEVERITY:-style}