Add missing comma from #1429

Turns out this is what was causing the OOM when building
binaryen since we were clobbering the EMSDK_NUM_CORES environment
variable by mistake.

emsdk.py

diff --git a/emsdk.py b/emsdk.py
index 8a64b3f..784c40d 100644
--- a/emsdk.py
+++ b/emsdk.py
@@ -2632,7 +2632,7 @@
   # of the SDK but we want to remove that from the current environment
   # if no such tool is active.
   # Ignore certain keys that are inputs to emsdk itself.
-  ignore_keys = set(['EMSDK_POWERSHELL', 'EMSDK_CSH', 'EMSDK_CMD', 'EMSDK_BASH', 'EMSDK_FISH'
+  ignore_keys = set(['EMSDK_POWERSHELL', 'EMSDK_CSH', 'EMSDK_CMD', 'EMSDK_BASH', 'EMSDK_FISH',
                      'EMSDK_NUM_CORES', 'EMSDK_NOTTY', 'EMSDK_KEEP_DOWNLOADS'])
   env_keys_to_add = set(pair[0] for pair in env_vars_to_add)
   for key in os.environ: