release-tools.sh: store the timestamp and release tag too

When maketgz invokes this script to generate the docs/RELEASE-TOOLS.md
file that gets bundled in the release, it now also passes

release-tools.sh: store the timestamp and release tag too

When maketgz invokes this script to generate the docs/RELEASE-TOOLS.md
file that gets bundled in the release, it now also passes on the exact
timestamp and version number so that those details also get mentioned in
the document. They will help users reproduce an identical tarball.

Closes #13319

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GHA: add shellcheck job and fix warnings, shell tidy-ups

Reviewed-by: Daniel Stenberg
Closes #13307

maketgz: put docs/RELEASE-TOOL.md into the tarball

Generated with scripts/release-tools.sh

The script lists the exact Debian package names and version numbers for
the tools that

maketgz: put docs/RELEASE-TOOL.md into the tarball

Generated with scripts/release-tools.sh

The script lists the exact Debian package names and version numbers for
the tools that are used to generate the tarball.

Closes #13239

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