fp6-img-plasma: both images are named after their shell

The Plasma Mobile archive was fp6-img.tar.xz and the Phosh one
fp6-img-phosh.tar.xz, so the unmarked one looked like the only one. Both
are now fp6-img-<shell>.tar.xz and extract to a directory of that name;
install.sh's messages and the README say so, and the README names the
dialer each shell calls with. The release upload takes dist/* and needs
no change.

.forgejo/workflows/build.yml

@@ -1,7 +1,7 @@
 name: image
 
 # Builds the flashable postmarketOS images for the Fairphone 6 (kernel from
-# milos-linux combined-stable + imsd) — Plasma Mobile (fp6-img.tar.xz) and
+# milos-linux combined-stable + imsd) — Plasma Mobile (fp6-img-plasma.tar.xz) and
 # Phosh (fp6-img-phosh.tar.xz) — and publishes both as the rolling 'latest'
 # release.
 #
@@ -40,7 +40,7 @@ jobs:
 
       # Second UI from the same package builds (the kernel and the aports
       # above are already current in pmbootstrap's work dir, so this is the
-      # install + export only): fp6-img-phosh.tar.xz next to fp6-img.tar.xz
+      # install + export only): fp6-img-phosh.tar.xz next to fp6-img-plasma.tar.xz
       # in the same release.
       - name: Build image (Phosh)
         run: FP6IMG_UI=phosh FP6IMG_KEEP_DIST=1 ./build.sh