build: improve process for govulncheck issue fixing

Quality of life improvements to make udpating go version for govulncheck issues:
- Allow to run govulncheck on master with workflow dispatch.
- Show image tag input when updating build image.

.github/workflows/govulncheck.yaml

@@ -5,6 +5,7 @@ on:
   schedule:
     - cron: '42 1 * * *'
   pull_request:
+  workflow_dispatch:
 
 permissions:
   contents: read

.github/workflows/kanister-image-build.yaml

@@ -1,5 +1,7 @@
 name: Update build image
 
+run-name: Update build image to ${{ inputs.tag }} as ${{ inputs.latest_tag }}
+
 permissions:
   contents: read