Publish desktop releases to a self-hosted dnf repo

scripts/rpm-publish.sh builds the RPM, signs it and the repo metadata
with a dedicated GPG key, and adds it to a dnf repo in a given directory,
keeping the newest three versions for downgrades. The directory holds
nothing secret, so it can be served as is.

Both publish scripts now share the version scheme in
scripts/lib/release-version.sh: <major>.<minor>.<commit count>, plain
enough for RPM, MSI and semver alike. Android's versionCode stays the
commit count.

Fixes the publish scripts failing under set -u when the repo URL
variable isn't set, which is every run after the first. tauri.conf.json
now declares the license and homepage for the package metadata.

src-tauri/tauri.conf.json

@@ -33,6 +33,8 @@
   "bundle": {
     "active": true,
     "targets": ["deb", "rpm"],
+    "license": "MIT",
+    "homepage": "https://code.garrettmills.dev/garrettmills/mtgc",
     "icon": [
       "icons/32x32.png",
       "icons/128x128.png",