That's the second time that I use etc/packager and this is a great help. Thank you @proux01 .
However, it does not seem to generate the right checksum for me
(twice I had to fix it by hand to release the opam packages).

First, I thought that I was a victim of macOS sed.
Maybe adding a SED=sed variable to packager could mitigate the issue with macOS (so that it is easier to set it to gsed--GNU sed).

However, this did not fix my problem.
Second, I thought that it was due to the URL of the archive.
As explained in the HOWTORELEASE https://github.com/math-comp/analysis/wiki/HOWTORELEASE
we need to download the archive once and put it back so that we do not rely on the URL.
As a consequence, what used to be
https://github.com/math-comp/analysis/archive/refs/tags/1.8.0.tar.gz
is now
https://github.com/math-comp/analysis/releases/download/1.8.0/analysis-1.8.0.tar.gz
I adjusted packager by changing the variable ARCHIVEURL to
ARCHIVEURL="https://github.com/math-comp/analysis/releases/download/$TAG/analysis-$TAG.tar.gz"
but for some reason I haven't figured out, the wrong checksum is still computed.

Which leads me to the conclusion that my shell skills are too limited. Do you have a clue of what I did wrong? @proux01