Support build of projects outside of beats directory

[jeniawhite]

What does this PR do?

There are beats that aren't part of the beats repository code-base.
Cloudbeat for example:

• https://github.com/elastic/cloudbeat

When calling the mage package that is based on the beats mage flows we are getting a failure:

Found Elastic Beats dir at /go/pkg/mod/github.com/elastic/beats/v7@v7.0.0-alpha2.0.20230718065235-46adbacfb5a8
exec: git "config" "--global" "--add" "safe.directory" "/go/pkg/mod/github.com/elastic/beats/v7@v7.0.0-alpha2.0.20230718065235-46adbacfb5a8"
>> build: Building cloudbeat-linux-amd64
exec: git "rev-parse" "HEAD"
fatal: detected dubious ownership in repository at '/go/src/github.com/elastic/cloudbeat'
To add an exception for this directory, call:
	git config --global --add safe.directory /go/src/github.com/elastic/cloudbeat
>>> Fixing file ownership issues from Docker at path=build/golang-crossbuild
chown took: 58.803µs, changed 1 files
>>> Fixing file ownership issues from Docker at path=build/golang-crossbuild/cloudbeat-linux-amd64
chown took: 13.401µs, changed 0 files
lstat build/golang-crossbuild/cloudbeat-linux-amd64: no such file or directory
Error: failed to expand template '-X github.com/elastic/beats/v7/libbeat/version.commit={{ commit }} -X github.com/elastic/beats/v7/libbeat/version.buildTime={{ date }}': template: inline:1:57: executing "inline" at <commit>: error calling commit: running "git rev-parse HEAD" failed with exit code 128
Error: failed building for linux/amd64: exit status 1
failed building for linux/amd64: exit status 1

I've investigated the code and saw that the safe.directory only triggers for beats directory:

	mountPoint, err := ElasticBeatsDir()
	if err != nil {
		return err
	}
	if err := sh.Run("git", "config", "--global", "--add", "safe.directory", mountPoint); err != nil {
		return err
	}

I've added the root path of the project as well.

Why is it important?

In order to support the building and packaging of beats that aren't part of the beats repository.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[botelastic]

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[elasticmachine]

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• Start Time: 2023-07-20T11:02:47.831+0000

• Duration: 71 min 0 sec

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Test	Results
Failed	0
Passed	27533
Skipped	2029
Total	29562

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🤖 GitHub comments

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To re-run your PR in the CI, just comment with:

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[jlind23]

LGTM

[michalpristas]

tested this, from my point of view it looks ok