This commit updates the cve2json GitHub workflow to add a branch
restriction for `main` branch, thus we don't need to spesify
`branch` input for `ad-m/github-push-action` step & change
the filepath for the `yaml2json.go` script.
The changes ensure that the workflow only runs on the `main`
branch & simply use `$GITHUB_WORKSPACE` variable as a filepath
for generating the JSON metadata of CVE templates.
f6b92f751a