Fix failure at manual pipeline run when leaving RELEASE variable empty
This failure happens because the variable precedence at GitLab prioratize manual pipeline run variables over generated ones during pipeline run [1] so when leaving the RELEASE variable empty, GitLab prioratize this empty value over the one generated by release detector script which causes the failure.
To fix that, this commit removes the RELEASE variable from Run pipeline
page, so it won't prefill it with empty value.
It also documents that the RELEASE variable is picked automatically in the README file.
Closes #341 (closed), Related to !486 (closed)
[1] https://salsa.debian.org/help/ci/variables/index.md#cicd-variable-precedence
Edited by Ahmed Siam