build: make poetry use existing lock file

This makes the build reproducible.

When `make dev-install` was run, it would ignore the existing
poetry lock file, install different versions of all the
libraries, then overwrite the lock file. Once `--no-update`
is added, it stops doing that, and installs exactly what is
in the poetry lock file.

Change-Id: If62637a40504d23deb47a05347a272e1c13bf41e
Signed-off-by: Charlie Bareham <charlie.bareham@arm.com>
diff --git a/tools/tlc/Makefile b/tools/tlc/Makefile
index 2de311b..e50b9dd 100644
--- a/tools/tlc/Makefile
+++ b/tools/tlc/Makefile
@@ -19,7 +19,7 @@
 
 .PHONY: dev-install
 dev-install:
-	poetry lock -n && poetry export --without-hashes > requirements.txt
+	poetry lock -n --no-update && poetry export --without-hashes > requirements.txt
 	poetry install -n
 	-poetry run mypy --install-types --non-interactive ./