Add plugins build command to makefile

[khanhtc1202]

What this PR does:

as title. Sample output

❯ make build/plugin PLUGINS=kubernetes
mkdir -p ~/.pipecd/plugins
Building plugins...
Building plugin: kubernetes
Plugins are built and copied to ~/.pipecd/plugins

Why we need it:

This command is needed while testing plugins with pipedv1

Which issue(s) this PR fixes:

Part of #4980

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 25.74%. Comparing base (74de8a0) to head (3fc60ce).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5402   +/-   ##
=======================================
  Coverage   25.74%   25.74%           
=======================================
  Files         449      449           
  Lines       48452    48452           
=======================================
  Hits        12474    12474           
  Misses      35005    35005           
  Partials      973      973           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Warashi]

Thanks. I commented on one nitpick

[Warashi]

LGTM