ci: use .cargo/config.toml

Setting rustflags to empty string avoids overwriting rustflags.
See https://github.com/actions-rust-lang/setup-rust-toolchain

Author: fewerner

.github/actions/make/action.yml

@@ -26,7 +26,6 @@ inputs:
   rust-rustflags:
     description: Set the value of RUSTFLAGS via setup-and-cache-rust
     required: false
-    default: -D warnings
 
 runs:
   using: composite

.github/actions/setup-and-cache-rust/action.yml

@@ -7,7 +7,7 @@ inputs:
   rustflags:
     description: Set the value of RUSTFLAGS (set to empty string to avoid overwriting existing flags)
     required: false
-    default: -D warnings
+    default: ''
   components:
     description: Additional components to install that don't come with the toolchain by default.
     required: false