fix: Fix plugin version output

[dsimansk]

@lance @rhuss

The following code should retrieve and set the actual plugin's version based on go.mod definition. The required change/enhancement on kn's midstream is to use require correctly.

It seems a bit cleaner than setting another set of compile flags. Anyway the current set of flags is set through main package, so we'd need to change that to pass flags during kn's build.

E.g. from my hacks:

require (
	github.com/boson-project/faas v0.9.0
)

replace (
	github.com/boson-project/faas => /home/dsimansk/Git/boson-project/faas
)

➜  client git:(release-v0.17.3) ✗ kn func version
v0.9.0

[rhuss]

looks good to me, but let's wait for @lance review.

[lance]

Seems like a good solution. Thanks @dsimansk !

[zroubalik]

LGTM