[piexwasm] Document how to activate our build release emsdk version

TL:DR; install and activate at the version number we use for releases.
See also CL:2435128. Comment only change.

Bug: 1132695
No-try: true
Change-Id: I8974c9aacae9d8e9455f225725d1855a66b1d096
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438552Reviewed-by: Alex Danilo <adanilo@chromium.org>
Reviewed-by: Austin Tankiang <austinct@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811959}

ui/file_manager/image_loader/piex/README.md

@@ -3,8 +3,8 @@ Install emscripten https://emscripten.org/docs/getting_started/index.html
 ```shell
   git clone https://github.com/emscripten-core/emsdk.git
   cd emsdk
-  ./emsdk install latest
-  ./emsdk activate latest
+  ./emsdk install 1.38.27
+  ./emsdk activate 1.38.27
   source ./emsdk_env.sh
 ```
 
@@ -30,7 +30,7 @@ Run tests: they must PASS
 Release: submit piex.js.wasm piex.out.wasm to the Chromium repository
 
 ```shell
-  git commit -a -m "Release piexwasm ..."
+  git commit -a -m "Release piexwasm at emscripten <version> ..."
   git cl upload
 ```