build: Use python -m to invoke pip when building docs (argoproj#9258)

Signed-off-by: juchao <juchao@coscene.io>

Makefile

@@ -639,7 +639,7 @@ docs-lint: /usr/local/bin/markdownlint
 	markdownlint docs --fix --ignore docs/fields.md --ignore docs/executor_swagger.md --ignore docs/cli --ignore docs/walk-through/the-structure-of-workflow-specs.md
 
 /usr/local/bin/mkdocs:
-	pip install mkdocs==1.2.4 mkdocs_material==8.1.9  mkdocs-spellcheck==0.2.1
+	python -m pip install mkdocs==1.2.4 mkdocs_material==8.1.9  mkdocs-spellcheck==0.2.1
 
 .PHONY: docs
 docs: /usr/local/bin/mkdocs \