emsdk

Do not mark cache generation as local (#1533)

local=True marks a repository rule to unconditionally re-execute anytime
bazel re-evaluates the workspace (almost every invocation) It works well
for processes that execute in <10ms, not large ones like the embuilder
cache generation.

I have tested this locally and it seems to work fine, but I'll be honest
that I don't understand the nuances of repository rules that well, nor
do I know much of anything about emscripten or the circumstances in
which the cache ought to be rebuilt.

I checked the `external/@emscripten_cache.marker` file that Bazel uses
to store the cache key for if it needs to re-evaluate, and changing any
of the options passed into register_emscripten_toolchains resulted in
the cache key changing, and it correctly resulted in the cache being
regenerated.

The big question I don't have the context to answer: are there files on
disk other than these two that need to be watched for changes, so the
repository rule should change if they change:
```
FILE:@@emscripten_bin_linux//BUILD.bazel
FILE:@@emsdk//emscripten_toolchain/default_config
```