nix: makes do.sh play better with nix

do.sh

@@ -4,30 +4,56 @@
 
 set -Eeuo pipefail # -e "Automatic exit from bash shell script on error"  -u "Treat unset variables and parameters as errors"
 
+checknix() {
+	if [[ "${DO_NIX_CUSTOM:=0}" -eq 1 ]]; then
+		echo "In an interactive nix env."
+	else
+		echo "Using poetry as runner, no nix detected."
+	fi
+}
+
 build() {
 	echo "I am ${FUNCNAME[0]}ing"
 	poetry build
 }
 
 fmt() {
+	if [[ "${DO_NIX_CUSTOM:=0}" -eq 1 ]]; then
+		black .
+	else
 		poetry run black .
+	fi
 	find . -type f -name "*.py" -exec sed -i -e 's/    /\t/g' {} \;
 }
 
 test() {
 	echo "I am ${FUNCNAME[0]}ing"
+	if [[ "${DO_NIX_CUSTOM:=0}" -eq 1 ]]; then
+		flake8 pyewjn tests
+		mypy pyewjn
+		pytest --benchmark-disable
+	else
 		poetry run flake8 pyewjn tests
 		poetry run mypy pyewjn
 		poetry run pytest --benchmark-disable
+	fi
 }
 
 benchmark() {
 	echo "I am ${FUNCNAME[0]}ing"
+	if [[ "${DO_NIX_CUSTOM:=0}" -eq 1 ]]; then
+		pytest --benchmark-only
+	else
 		poetry run pytest --benchmark-only
+	fi
 }
 
 htmlcov() {
+	if [[ "${DO_NIX_CUSTOM:=0}" -eq 1 ]]; then
+		pytest --cov-report=html
+	else
 		poetry run pytest --cov-report=html
+	fi
 }
 
 release() {