Makefile: optimize jsonnet-format target

[daixiang0]

Signed-off-by: Xiang Dai [email protected]

Fix following error:

# make examples
find . -name 'vendor' -prune -o -name '*.libsonnet' -print -o -name '*.jsonnet' -print | \
	xargs -n 1 -- jsonnetfmt -n 2 --max-blank-lines 2 --string-style s --comment-style s -i
xargs: jsonnetfmt: No such file or directory

[kakkoyun]

Sorry for this inconvenience, I've forgotten this bit.
I've already fixed this, in one of our projects so have a look at it.

https://github.com/observatorium/observatorium/blob/master/Makefile

[daixiang0]

Since even we download binary, it may not work as expected, i would check the cmd and comment here.