Use dynamic npx path detection instead of hardcoded paths

Replace hardcoded /usr/bin/npx and /usr/local/bin/npx with $(which npx)
to handle different npx installation locations.

Co-authored-by: Ona <no-reply@ona.com>

Author: jespino

gitpod/gitpod.Dockerfile

@@ -3,11 +3,12 @@ FROM gitpod/workspace-full:latest
 # Disable npm lifecycle scripts and npx for security
 RUN npm config set ignore-scripts true --location=user && \
     echo 'ignore-scripts true' >> ~/.yarnrc && \
-    rm -f /usr/bin/npx /usr/local/bin/npx && \
-    echo '#!/bin/sh' > /usr/local/bin/npx && \
-    echo 'echo "npx is disabled for security reasons. Use explicit package installation instead." >&2' >> /usr/local/bin/npx && \
-    echo 'exit 1' >> /usr/local/bin/npx && \
-    chmod +x /usr/local/bin/npx
+    NPX_PATH=$(which npx) && \
+    rm -f "$NPX_PATH" && \
+    echo '#!/bin/sh' > "$NPX_PATH" && \
+    echo 'echo "npx is disabled for security reasons. Use explicit package installation instead." >&2' >> "$NPX_PATH" && \
+    echo 'exit 1' >> "$NPX_PATH" && \
+    chmod +x "$NPX_PATH"
 
 # Cache firebase
 RUN npm install --global --ignore-scripts npm firebase firebase-tools