Switch to `HEAD.zip` when downloading archive from github. (#806)

This works regardless of the name of the primary branch so that
the code will continue to work if/when we rename `main` to `master`.

See: #805
diff --git a/emsdk.py b/emsdk.py
index 666e507..113027b 100644
--- a/emsdk.py
+++ b/emsdk.py
@@ -44,7 +44,10 @@
 emscripten_releases_download_url_template = "https://storage.googleapis.com/webassembly/emscripten-releases-builds/%s/%s/wasm-binaries.%s"
-emsdk_zip_download_url = 'https://github.com/emscripten-core/emsdk/archive/master.zip'
+# This was previously `master.zip` but we are transitioning to `main` and
+# `HEAD.zip` works for both cases.  In future we could switch this to
+# `main.zip` perhaps.
+emsdk_zip_download_url = 'https://github.com/emscripten-core/emsdk/archive/HEAD.zip'
 zips_subdir = 'zips/'