fix(ci): quote parameterized image: values in build-all-versions workflows

Strict YAML parsers reject the colon separating ${OS} and ${OS_VERSION}
in an unquoted scalar (it looks like a mapping value). Wrap the whole
image value in double quotes to make it an unambiguous string.

Mirrors the convention already used by
build-r-minor-sensitive-packages.yaml.

build-all-versions-install-deps.yaml

@@ -17,7 +17,7 @@
 
 # steps:
 #   - name: 'Install common R deps'
-#     image: reg.devxy.io/rpkgs/build-env-${OS}:${OS_VERSION}
+#     image: "reg.devxy.io/rpkgs/build-env-${OS}:${OS_VERSION}"
 #     pull: true
 #     environment:
 #       REPO_RO_TOKEN: