Find the emsdk by relative path so that this works with the monorepo. (#40207)

Find the emsdk by relative path so that this works with the monorepo.

tools/activate_emsdk.py

diff --git a/tools/activate_emsdk.py b/tools/activate_emsdk.py
index 30fd1b0..9b4e3ae 100644
--- a/tools/activate_emsdk.py
+++ b/tools/activate_emsdk.py
@@ -9,7 +9,9 @@
 import subprocess
 import sys
 
-EMSDK_ROOT = os.path.join('src', 'buildtools', 'emsdk')
+EMSDK_ROOT = os.path.abspath(
+    os.path.join(__file__, '..', '..', '..', 'buildtools', 'emsdk')
+)
 
 EMSDK_PATH = os.path.join(EMSDK_ROOT, 'emsdk.py')