Only add node/bin to user's PATH if one is not already found

If the user already has a version of node in their PATH don't clobber
it.  This doesn't effect emscripten since the version of node we use
there is controlled via the config file, not via PATH.

Part of fix for #705.

emsdk.py

diff --git a/emsdk.py b/emsdk.py
index 8ae6b07..17b8506 100644
--- a/emsdk.py
+++ b/emsdk.py
@@ -1433,6 +1433,8 @@
   path_add = [to_native_path(EMSDK_PATH)]
   for tool in active_tools:
     if hasattr(tool, 'activated_path'):
+      if hasattr(tool, 'activated_path_skip') and which(tool.activated_path_skip):
+        continue
       path = to_native_path(tool.expand_vars(tool.activated_path))
       path_add.append(path)
   return path_add

emsdk_manifest.json

diff --git a/emsdk_manifest.json b/emsdk_manifest.json
index 927cdd9..b14385b 100644
--- a/emsdk_manifest.json
+++ b/emsdk_manifest.json
@@ -204,6 +204,7 @@
     "arch": "x86",
     "windows_url": "node-v14.15.5-win-x86.zip",
     "activated_path": "%installation_dir%/bin",
+    "activated_path_skip": "node",
     "activated_cfg": "NODE_JS='%installation_dir%/bin/node%.exe%'",
     "activated_env": "EMSDK_NODE=%installation_dir%/bin/node%.exe%"
   },
@@ -214,6 +215,7 @@
     "bitness": 32,
     "linux_url": "node-v14.15.5-linux-armv7l.tar.xz",
     "activated_path": "%installation_dir%/bin",
+    "activated_path_skip": "node",
     "activated_cfg": "NODE_JS='%installation_dir%/bin/node%.exe%'",
     "activated_env": "EMSDK_NODE=%installation_dir%/bin/node%.exe%"
   },
@@ -226,6 +228,7 @@
     "windows_url": "node-v14.15.5-win-x64.zip",
     "linux_url": "node-v14.15.5-linux-x64.tar.xz",
     "activated_path": "%installation_dir%/bin",
+    "activated_path_skip": "node",
     "activated_cfg": "NODE_JS='%installation_dir%/bin/node%.exe%'",
     "activated_env": "EMSDK_NODE=%installation_dir%/bin/node%.exe%"
   },
@@ -237,6 +240,7 @@
     "macos_url": "node-v14.15.5-darwin-x64.tar.gz",
     "linux_url": "node-v14.15.5-linux-arm64.tar.xz",
     "activated_path": "%installation_dir%/bin",
+    "activated_path_skip": "node",
     "activated_cfg": "NODE_JS='%installation_dir%/bin/node%.exe%'",
     "activated_env": "EMSDK_NODE=%installation_dir%/bin/node%.exe%"
   },