Make sourcehut namespace configurable

Replace hardcoded ~pmos namespace with an actually configurable namespace. The namespace shows up in sr.ht links, and while it works to put any value there, as long as the ID is right, it leads to confusion to have it not pointing to the user that is running the test.

I have named the variable "namespace" and included the ~ character, because sr.ht is getting support for something like organizations at some point. Maybe then setting another character is required.

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@MartijnBraam: for some reason, it ignores whatever I put with SRHT_NAMESPACE in .env, and uses the default ~user-not-configured. Can you please take a look at the patch and tell me what I'm doing wrong?