Fix new_warning_with_corrupt_ws missing "USER".

Also removed USER in all invocations to avoid this in the future.

tests/testsuite/support/mod.rs

@@ -1679,6 +1679,7 @@ fn _process(t: &OsStr) -> cargo::util::ProcessBuilder {
         .env_remove("XDG_CONFIG_HOME") // see #2345
         .env("GIT_CONFIG_NOSYSTEM", "1") // keep trying to sandbox ourselves
         .env_remove("EMAIL")
+        .env_remove("USER") // not set on some rust-lang docker images
         .env_remove("MFLAGS")
         .env_remove("MAKEFLAGS")
         .env_remove("CARGO_MAKEFLAGS")

tests/testsuite/workspaces.rs

@@ -992,6 +992,7 @@ root: [..]
 fn new_warning_with_corrupt_ws() {
     let p = project().file("Cargo.toml", "asdf").build();
     p.cargo("new bar")
+        .env("USER", "foo")
         .with_stderr(
             "\
 [WARNING] compiling this new crate may not work due to invalid workspace configuration