Fix shell variable assignment and update shell activation command in Makefile

Makefile

@@ -2,7 +2,7 @@ TARGET=thechart
 VERSION=1.0.0
 ROOT=/home/will
 ICON=chart-671.png
-SHELL=/bin/fish
+SHELL=fish
 help: ## Show this help
 	@grep -E -h '\s##\s' $(MAKEFILE_LIST) | sort | awk 'BEGIN {FS = ":.*?## "}; {printf "\033[36m%-20s\033[0m %s\n", $$1, $$2}'
 install: ## Set up the development environment
@@ -47,7 +47,7 @@ attach: ## Open a shell in the container
 	docker-compose exec -it ${TARGET} /bin/bash
 shell: ## Open a shell in the local environment
 	@echo "Opening a shell in the local environment..."
-	${SHELL} -c "eval (poetry env activate)"
+	source ./.venv/bin/activate.${SHELL} && /bin/${SHELL}
 requirements: ## Export the requirements to a file
 	@echo "Exporting requirements to requirements.txt..."
 	poetry export --without-hashes -f requirements.txt -o requirements.txt