fix(pipeline): validate presence of tag on tag events to avoid panic

[ecrupper]

Due to how the environment is set up, the exec pipeline command needs to have a tag value on a tag event.

$ vela exec pipeline --pipeline-type go --event tag
panic: runtime error: index out of range [1] with length 1

goroutine 1 [running]:
github.com/go-vela/types/library.(*Build).Environment(0xc0006d7560, {0x2cc080e, 0x9}, {0x2cb70f5, 0x4})
	/Users/Z004X4Y/go/pkg/mod/github.com/go-vela/[email protected]/library/build.go:198 +0x31dd
github.com/go-vela/server/compiler/native.environment(0xc000606968?, 0x0, 0xc0002b81a0, 0x0)
	/Users/Z004X4Y/go/pkg/mod/github.com/go-vela/[email protected]/compiler/native/environment.go:326 +0xaec
github.com/go-vela/server/compiler/native.(*client).EnvironmentBuild(0xc0002204b0)
.....

Changed to:

$ vela exec pipeline --pipeline-type go --event tag 
FATA[0000] no tag provided for tag event  

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (08b96be) 82.23% compared to head (21b602a) 82.24%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #529      +/-   ##
==========================================
+ Coverage   82.23%   82.24%   +0.01%     
==========================================
  Files         147      147              
  Lines        3602     3605       +3     
==========================================
+ Hits         2962     2965       +3     
  Misses        473      473              
  Partials      167      167              

Files	Coverage Δ
action/pipeline/validate.go	86.20% <100.00%> (+0.49%)	⬆️