Agent: Fix linux command line for http request

[ilija-lazoroski]

What does this PR do?

Fixes linux Communicate as new user PBA.
The post breach action was failing previously because it relays on shell=True.
I was replaced with a check for the executables.

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?

Testing Checklist

• Added relevant unit tests?
• Have you successfully tested your changes locally? Elaborate:

  Tested by {Running the Monkey locally with relevant config/running Island/...}

• If applicable, add screenshots or log transcripts of the feature working

Explain Changes

Are the commit messages enough? If not, elaborate.

[codecov]

Codecov Report

Merging #1407 (4a240ab) into release/1.11.0 (babaadf) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

❗ Current head 4a240ab differs from pull request most recent head 535fa8a. Consider uploading reports for the commit 535fa8a to get more accurate results

@@ Coverage Diff @@ ## release/1.11.0 #1407 +/- ## ================================================== - Coverage 40.08% 40.07% -0.01%  ================================================== Files 465 465 Lines 13842 13845 +3 ================================================== Hits 5548 5548 - Misses 8294 8297 +3 

Impacted Files	Coverage Δ
...key/post_breach/actions/communicate_as_new_user.py	0.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update babaadf...535fa8a. Read the comment docs.