v1.2.3: move CI to self-hosted runners + rust-cache

Linux / Android / mipsel build jobs now run on two self-hosted runners
on a Hetzner 8-core / 31 GB Ubuntu 24.04 box with Rust, Android SDK+NDK
r26c, all cross-compile toolchains and Docker pre-installed. macOS and
Windows still run on GitHub-hosted — we don't self-host those OSes and
the free minutes on a public repo are plenty.

Adds Swatinem/rust-cache@v2 to every cargo-using job so target/ + cargo
registry survive between runs. With warm caches the Linux jobs take
~1min each and the Android job ~3-4min; cold runs are ~9min for
Android and ~2min for everything else. Release wall time before this
change was ~13m consistently; it should now sit around 6-7m.

No new user-facing code in this release — primarily an infra change
exercised by an actual tag-push so we verify the full pipeline works
end-to-end from the new runners.

android/app/build.gradle.kts

@@ -14,8 +14,8 @@ android {
         applicationId = "com.therealaleph.mhrv"
         minSdk = 24 // Android 7.0 — covers 99%+ of live devices.
         targetSdk = 34
-        versionCode = 122
-        versionName = "1.2.2"
+        versionCode = 123
+        versionName = "1.2.3"
 
         // Ship all four mainstream Android ABIs:
         //   - arm64-v8a      — 95%+ of real-world Android phones since 2019