build: Fix flatpak CI failing

Build libstemmer as a shared library. Use current mainline since that
at least supports build system args.

Shared lib patch courtesy of Stefano Rivera <stefanor@debian.org>,
updated to current head.
3 jobs for mjog/334-libstemmer in 28 minutes and 30 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #902140
fedora

00:08:05

passed #902141
ubuntu

00:13:25

 
  Test
passed #902142
flatpak
flatpak

00:15:04