xtask: force --target-applies-to-host when running Clippy check

Without this, the configs from `.cargo/config.toml` were not read in CI
tasks, causing false positives when running Clippy on CI (i.e. there
were issues observed when compiling locally that were not found when
compiling remotely).

Not entirely sure why it's needed, because I'm seeing the issues when
I'm using `cargo xtask ci clippy` locally, with nothing changed.

xtask/src/ci.rs

@@ -179,20 +179,27 @@ fn check_typos() -> Result<()> {
 
 fn check_clippy() -> Result<()> {
     cmd!("rustup run {NIGHTLY} cargo clippy --all-targets --features testing -- -D warnings")
+        // Work around https://github.com/rust-lang/cargo/issues/10744
+        .env("CARGO_TARGET_APPLIES_TO_HOST", "true")
         .run()?;
+
     cmd!(
         "rustup run {NIGHTLY} cargo clippy --workspace --all-targets
             --exclude matrix-sdk-crypto --exclude xtask
             --no-default-features
             --features native-tls,experimental-sliding-sync,sso-login,testing
             -- -D warnings"
     )
+    .env("CARGO_TARGET_APPLIES_TO_HOST", "true")
     .run()?;
+
     cmd!(
         "rustup run {NIGHTLY} cargo clippy --all-targets -p matrix-sdk-crypto
             --no-default-features -- -D warnings"
     )
+    .env("CARGO_TARGET_APPLIES_TO_HOST", "true")
     .run()?;
+
     Ok(())
 }