execcmd: fix compilation errors on mac

[nodir-t]

Summary

This PR fixes compilation errors in agent/engine/execcmd package on Mac.

Implementation details

manager_start.go is implemented only for linux and windows, however manager_start_test.go and manager_init_task_test.go currently are not limited to those OSes. In particular, the don't compile on Mac.

Update their build tags to require linux || windows too.

Testing

cd agent
go test -tags unit ./engine/execcmd/

New tests cover the changes: no

Description for the changelog

Fix compilation errors in execcmd package on Mac.

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[angelcar]

Seems like this is failing static checks + mac tests

[nodir-t]

Static checks are passing now. Mac testes were removed from this PR (rebased).

[fierlion]

closing for now -- will reopen as part of future work