Use system pip3 rather than venv pip3

Following opensafely-core/research-template-docker#26
which added venv activation into .bashrc for the rstudio user (the default
user for our codespaces/devcontainer configuration), calling `pip3` would
call /opt/venv/bin/pip3. Within a virtualenv, the `--user` option is not
allowed and users reported this error. Instead we decouple the installation
of opensafely-cli from the venv so it is always available regardless of
virtualenv state.

.devcontainer/postCreate.sh

@@ -2,7 +2,7 @@
 
 set -euo pipefail 
 
-pip3 install --user -r .devcontainer/requirements.in
+/usr/local/bin/pip3 install --user -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\")"

analysis/stata.do

@@ -0,0 +1 @@
+capture noisily import delimited ./output/output/dataset.csv, clear