Remove --user from pip install in postCreate.sh

venv is activated prior to this script execution following
opensafely-core/research-template-docker#26
so --user installation no longer possible

.devcontainer/postCreate.sh

@@ -2,7 +2,7 @@
 
 set -euo pipefail 
 
-pip3 install --user -r .devcontainer/requirements.in
+pip3 install -r .devcontainer/requirements.in
 
 #set R working directory
 ! grep -q "$1" $R_HOME/etc/Rprofile.site && sudo tee -a $R_HOME/etc/Rprofile.site <<< "setwd(\"$1\")"