License under MIT

No licence file meant nobody had permission to redistribute this, which is a
strange position for a public repo with an AUR package pointing at it. The
PKGBUILD said license=('custom') because there was nothing truthful to put
there.

MIT is not one of the licences Arch keeps in /usr/share/licenses/common, so
the package now fetches the licence from its own tag and installs a copy. The
release workflow checksums that copy from the tag it checked out, rather than
trusting whatever main has drifted to.

Declared in package.json and Cargo.toml too, so the manifests and the package
agree on the answer.

src-tauri/Cargo.toml

@@ -4,6 +4,7 @@ version = "0.0.1"
 description = "A calendar for Google Calendar"
 authors = ["Margin"]
 edition = "2021"
+license = "MIT"
 
 [lib]
 name = "margin_calendar_lib"