Inherit RUSTC_BOOTSTRAP when testing wasm

This is implemented with the freshly-released Wasmtime 19 and should
prevent beta breakage from wasm tests that was observed and fixed
in #122640 again.

src/bootstrap/src/lib.rs

@@ -1389,6 +1389,13 @@ impl Build {
         if let Some(path) = finder.maybe_have("wasmtime") {
             if let Ok(mut path) = path.into_os_string().into_string() {
                 path.push_str(" run -C cache=n --dir .");
+                // Make sure that tests have access to RUSTC_BOOTSTRAP. This (for example) is
+                // required for libtest to work on beta/stable channels.
+                //
+                // NB: with Wasmtime 20 this can change to `-S inherit-env` to
+                // inherit the entire environment rather than just this single
+                // environment variable.
+                path.push_str(" --env RUSTC_BOOTSTRAP");
                 return Some(path);
             }
         }

src/ci/docker/host-x86_64/test-various/Dockerfile

@@ -55,9 +55,9 @@ ENV RUST_CONFIGURE_ARGS \
 ENV NO_DEBUG_ASSERTIONS=1
 ENV NO_OVERFLOW_CHECKS=1
 
-RUN curl -L https://github.com/bytecodealliance/wasmtime/releases/download/v18.0.2/wasmtime-v18.0.2-x86_64-linux.tar.xz | \
+RUN curl -L https://github.com/bytecodealliance/wasmtime/releases/download/v19.0.0/wasmtime-v19.0.0-x86_64-linux.tar.xz | \
   tar -xJ
-ENV PATH "$PATH:/wasmtime-v18.0.2-x86_64-linux"
+ENV PATH "$PATH:/wasmtime-v19.0.0-x86_64-linux"
 
 ENV WASM_TARGETS=wasm32-wasip1
 ENV WASM_SCRIPT python3 /checkout/x.py --stage 2 test --host='' --target $WASM_TARGETS \