Example fails to build, missing emcc.py

[sfc-gh-aminyaylov]

The wasm build rule expects emscripten binaries to be available in the sandbox directory external/emsdk/emscripten/emcc.py.

The build runs @proxy_wasm_cpp_sdk proxy_wasm_cpp_sdk_dependencies_extra(), which in turn runs @emsdk emscripten_deps(), which registers the emscripten archive, which should make emcc.py available in the expected sandbox directory.

However, since bazel uses lazy evaluation, and there is no rule directly using the emscripten_bin_linux archive, this filegroup is never instantiated. I'm not clear how this ever worked.

Simple repro from head:

$ git clone git@github.com:proxy-wasm/proxy-wasm-cpp-sdk.git
$ cd proxy-wasm-cpp-sdk
$ bazel build example:http_wasm_example.wasm
INFO: Analyzed target //example:http_wasm_example.wasm (25 packages loaded, 222 targets configured).
INFO: Found 1 target...
ERROR: /home/user/proxy-wasm-cpp-sdk/BUILD:32:11: Compiling proxy_wasm_intrinsics.cc failed: (Exit 2): emcc.sh failed: error executing command external/emsdk/emscripten_toolchain/emcc.sh '--sysroot=external/emsdk/emscripten/cache/sysroot' -fdiagnostics-color -fno-exceptions -fno-strict-aliasing -funsigned-char -no-canonical-prefixes ... (remaining 29 argument(s) skipped)

Use --sandbox_debug to see verbose messages from the sandbox
python3: can't open file '/home/user/.cache/bazel/_bazel_user/63d72b24569786f1ee6718e6de54cbab/sandbox/linux-sandbox/2/execroot/proxy_wasm_cpp_sdk/external/emsdk/emscripten/emcc.py': [Errno 2] No such file or directory
Target //example:http_wasm_example.wasm failed to build
Use --verbose_failures to see the command lines of failed build steps.
INFO: Elapsed time: 4.933s, Critical Path: 0.05s
INFO: 11 processes: 11 internal.
FAILED: Build did NOT complete successfully