webpack: Fix CSS source map generation on 1-CPU systems.
webpack: Fix CSS source map generation on 1-CPU systems.

We were passing a SourceMapGenerator as `map`, but it seems that
css-minimizer-webpack-plugin expects a string, and only implicitly
stringifies it when running with parallelism.

Fixes #18727.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

eeshangarg/zulip,andersk/zulip,andersk/zulip,hackerkid/zulip,kou/zulip,kou/zulip,hackerkid/zulip,zulip/zulip,andersk/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,hackerkid/zulip,rht/zulip,punchagan/zulip,zulip/zulip,rht/zulip,kou/zulip,kou/zulip,rht/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,andersk/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,zulip/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,andersk/zulip,kou/zulip,kou/zulip,eeshangarg/zulip