Update RUSTFLAGS warning to suggest CARGO_BUILD_RUSTFLAGS (#2252)

Modify the warning message when RUSTFLAGS is set to recommend using
CARGO_BUILD_RUSTFLAGS instead, which the CLI can merge with remapping.

Author: leighmcculloch

cmd/soroban-cli/src/commands/contract/build.rs

@@ -507,7 +507,7 @@ fn make_rustflags_to_remap_absolute_paths(print: &Print) -> Result<Option<String
     }
 
     if env::var("RUSTFLAGS").is_ok() {
-        print.warnln("`RUSTFLAGS` set. Dependency paths will not be remapped; builds may not be reproducible.");
+        print.warnln("`RUSTFLAGS` set. Dependency paths will not be remapped; builds may not be reproducible. Use CARGO_BUILD_RUSTFLAGS instead, which the CLI will merge with remapping.");
         return Ok(None);
     }