chore: instruct pipenv to put venv into project dir

This will make it a bit easier to configure caching in gitlab CI.

.gitignore

@@ -7,3 +7,4 @@ __pycache__
 .vscode
 sinologie-anki-pack-*
 .build-cache
+.venv

Makefile

@@ -71,7 +71,7 @@ $(RELEASE_ARCHIVE): $(RELEASE_DIR)
 $(RELEASE_DIR): $(BUILT_TEMPLATES_DIR) $(APKGS) $(RELEASED_STATIC_CONTENT)
 
 $(PIP_INSTALLED_FILE): Pipfile Pipfile.lock
-	pipenv install
+	PIPENV_VENV_IN_PROJECT=1 pipenv install
 	mkdir -p $(CACHE_DIR)
 	touch $(PIP_INSTALLED_FILE)
 
@@ -81,7 +81,7 @@ $(YARN_INSTALLED_FILE): package.json yarn.lock
 	touch $(PIP_INSTALLED_FILE)
 
 $(APKGS): $(PIP_INSTALLED_FILE) $(BUILT_TEMPLATES_DIR) $(CONTENT_SOURCES)
-	pipenv run build/export_apkgs.py -c $(CONTENT_DIR) -t $(BUILT_TEMPLATES_DIR) -o $(RELEASE_DIR)
+	PIPENV_VENV_IN_PROJECT=1 pipenv run build/export_apkgs.py -c $(CONTENT_DIR) -t $(BUILT_TEMPLATES_DIR) -o $(RELEASE_DIR)
 
 $(BUILT_TEMPLATES_DIR): $(TEMPLATE_SOURCES)
 	@yarn run parcel build --no-source-maps --dist-dir=$(RELEASE_DIR) $(PARCEL_RELEASE_ENTRY_HTMLS)