Explicitly set envs_dir during builds to avoid cross-talk between existing environments on shared builds (!8)

This MR explicitly sets this variable on-the-fly to avoid cross-talking. Closes #12 (closed).

doc/macos-ci-install/setup-paths.sh

@@ -6,3 +6,11 @@ set -x
 sed -e '/^\/usr\/local/d' -i .orig /etc/paths
 sed -e '/^\/Library\/TeX\/texbin/d' -i .orig /etc/paths
 echo -e "/usr/local/bin\n/usr/local/sbin\n/usr/local/opt/coreutils/libexec/gnubin\n/Library/TeX/texbin\n$(cat /etc/paths)" > /etc/paths
+
+# ensures no cross-talking happens between miniconda installations on
+# the shared builder - see bob/bob.devtools#12 and bob/bob.devtools!8
+condadir=/Users/${USER}/.conda
+mkdir ${condadir}
+touch ${condadir}/environments.txt
+chown -R ${USER}:staff ${condadir}
+chmod a-w ${condadir}/environments.txt