`--host` flag in search command wants registry index URL, not registry website host

[fa7ca7]

The issue is the same as in #3797, but relates to bin/search.

[ashleygwilliams]

for people interested in taking this on, steps required would include, but may not be limited to:

• deprecate --host
• rename it as --index

a good example is this PR which does the same thing, but for publish #4209

(thanks for filing @dethoter! 🐶 ⭐️ 🌴 )