ci: revise Rust caching configuration (#10512)

We primarily rely on sccache for speeding up our Rust builds. The
`setup-rust-toolchain` action configures `rust-cache` in addition but
tries to set it up for the root of the repository which doesn't work in
our scenario. It is also unnecessary to set up additional caching for
the `target` directory because we use `sccache` anyway.

sccache only caches compile artifacts but it can also be beneficial to
cache the downloaded Rust sources. Those aren't very big so we use the
`rust-cache` action for that but disable the `target` cache. This config
option isn't forwarded by `setup-rust-toolchain` so we disable its cache
integration and use `rust-cache` directly.

.github/workflows/_rust.yml

@@ -11,10 +11,8 @@ permissions:
   contents: "read"
   id-token: "write"
 
-# Never tolerate warnings. Duplicated in `_tauri.yml`
 env:
-  RUSTFLAGS: "-Dwarnings --cfg tokio_unstable"
-  RUSTDOCFLAGS: "-D warnings"
+  RUSTFLAGS: "--cfg tokio_unstable"
 
 jobs:
   static-analysis:

.github/workflows/_tauri.yml

@@ -7,10 +7,7 @@ defaults:
   run:
     working-directory: ./rust/gui-client
 
-# Never tolerate warnings. Source of truth is `_rust.yml`
 env:
-  RUSTFLAGS: "-Dwarnings"
-  RUSTDOCFLAGS: "-D warnings"
   TSLINK_BUILD: true
 
 permissions: