jhbuild plugin: allow running as 'root'

[lucyllewy]

JHBuild has recently acquired the ability to run under the root account with an environment variable to acknowledge the operation.

If applied, this commit will:

• Add JHBUILD_RUN_AS_ROOT environment variable to jhbuild plugin
• Remove previous attempt to work around snapcraft running under root from jhbuild plugin

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• If this is a bugfix. Have you checked that there is a bug report open for the issue you are trying to fix on bug reports?
• If this is a new feature. Have you discussed the design on the forum?
• Have you successfully run ./runtests.sh static?
• Have you successfully run ./runtests.sh unit?
  • FAILED (failures=3, errors=5)
  • failures and errors are in unrelated parts of snapcraft:
    • tests.unit.sources.test_bazaar.BazaarDetailsTestCase.test_bzr_details_commit
    • tests.unit.sources.test_bazaar.BazaarDetailsTestCase.test_bzr_details_tag
    • tests.unit.sources.test_mercurial.MercurialDetailsTestCase.test_hg_details_branch
    • tests.unit.sources.test_mercurial.MercurialDetailsTestCase.test_hg_details_commit
    • tests.unit.sources.test_mercurial.MercurialDetailsTestCase.test_hg_details_tag
    • tests.unit.test_elf.TestPatcher.test_tools_from_snap_used_if_using_snap
    • tests.unit.test_elf.TestPatcherErrors.test_patch_uses_snapped_strip
    • tests.unit.test_lifecycle.CoreSetupTestCase.test_core_setup_if_docker_env

---

[codecov-io]

Codecov Report

Merging #2141 into master will increase coverage by 0.27%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2141      +/-   ##
==========================================
+ Coverage   91.01%   91.29%   +0.27%     
==========================================
  Files         193      193              
  Lines       12181    12136      -45     
  Branches     1820     1810      -10     
==========================================
- Hits        11087    11080       -7     
+ Misses        752      714      -38     
  Partials      342      342

Impacted Files	Coverage Δ
snapcraft/plugins/jhbuild.py	41.66% <0%> (+10.04%)	⬆️

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[kyrofa]

I'm afraid my lack of jhbuild experience means I can only give this a cosmetic review. @jhenstridge, any chance you could give this a look when you have a moment?

[jhenstridge]

It looks fine to me. The ratio of lines removed to lines added is a good sign. From the look of it, the existing integration test should verify that this all works as expected.