tests: execute dogfood tests with incremental compilation disabled

reduces target/ dir size of when "cago test"ing by around 2 gigs.

tests/dogfood.rs

@@ -12,6 +12,7 @@ fn dogfood() {
     let output = std::process::Command::new(clippy_cmd)
         .current_dir(root_dir)
         .env("CLIPPY_DOGFOOD", "1")
+        .env("CARGO_INCREMENTAL", "0")
         .arg("clippy-preview")
         .arg("--all-targets")
         .arg("--all-features")
@@ -50,6 +51,7 @@ fn dogfood_tests() {
         let output = std::process::Command::new(&clippy_cmd)
             .current_dir(root_dir.join(d))
             .env("CLIPPY_DOGFOOD", "1")
+            .env("CARGO_INCREMENTAL", "0")
             .arg("clippy")
             .arg("--")
             .args(&["-D", "clippy::all"])