upgrade: Set an explicit value for PATH.

Previous versions of zulip used `nvm alias default ...` to have `nvm`
prepend the full path to the latest `node` install to the `PATH` in
root's shell.  Unfortunately, this means that `update-prod-static`,
when called from `upgrade-zulip-stage-2` after an upgrade of node in
`install-node`, would still have the full path to the _old_ `node` at
the start of its PATH, because the PATH of `upgrade-zulip-stage-2`
would still be unchanged.

Bootstrap out of this by setting a known-reasonable PATH during
upgrade, and remove the problematic `nvm alias default` behaviour.

Fixes #18258.

scripts/lib/install-node

@@ -45,8 +45,8 @@ if [ "$current_node_version" != "v$node_version" ] || ! [ -L "$node_wrapper_path
         . "$NVM_DIR/nvm.sh"
     fi
 
-    nvm install "$node_version" && nvm alias default "$node_version"
-    NODE_BIN="$(nvm which default)"
+    nvm install "$node_version"
+    NODE_BIN="$(nvm which $node_version)"
 
     # Fix messed-up uid=500 and group write bits produced by nvm
     n=${NODE_BIN%/bin/node}
@@ -57,6 +57,12 @@ if [ "$current_node_version" != "v$node_version" ] || ! [ -L "$node_wrapper_path
     ln -nsf "$NODE_BIN" /usr/local/bin/node
     ln -nsf "$(dirname "$NODE_BIN")/npm" /usr/local/bin/npm
     ln -nsf "$(dirname "$NODE_BIN")/npx" /usr/local/bin/npx
+
+    # Tell NVM that we don't want it messing around with $PATH, which
+    # can get us into trouble, if we've just upgraded but our parent
+    # env still has the old version first in its PATH.  Tell it to use
+    # the newly-symlinked one that it will find in /usr/local/bin
+    nvm alias default system
 fi
 
 # Work around the fact that apparently sudo doesn't clear the HOME

scripts/lib/upgrade-zulip-stage-2

@@ -37,6 +37,9 @@ from scripts.lib.zulip_tools import (
 
 assert_running_as_root()
 
+# Set a known, reliable PATH
+os.environ["PATH"] = "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
+
 logging.Formatter.converter = time.gmtime
 logging.basicConfig(format="%(asctime)s upgrade-zulip-stage-2: %(message)s", level=logging.INFO)