Fix tasks to use `Set-SamplerTaskVariable`

[johlju]

Pull Request

Pull Request (PR) description

• Tasks now uses Set-SamplerTaskVariable from the module Sampler to set the common build task variables.

This Pull Request (PR) fixes the following issues

• Fixes #19
• Fixes #82

Task list

• [x] Added an entry to the change log under the Unreleased section of the file CHANGELOG.md. Entry should say what was changed and how that affects users (if applicable), and reference the issue being resolved (if applicable).
• [ ] Documentation added/updated in README.md.
• [ ] Comment-based help added/updated.
• [ ] Localization strings added/updated in all localization files as appropriate.
• [ ] Examples appropriately added/updated.
• [x] Unit tests added/updated.
• [ ] Integration tests added/updated (where possible).
• [x] New/changed code adheres to DSC Community Style Guidelines.

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This change is 

[codecov[bot]]

Codecov Report

Merging #118 (76c3639) into main (0de5fe4) will increase coverage by 0%. The diff coverage is 100%.

@@         Coverage Diff         @@
##           main   #118   +/-   ##
===================================
  Coverage    97%    97%           
===================================
  Files        42     42           
  Lines       966    882   -84     
===================================
- Hits        945    864   -81     
+ Misses       21     18    -3     

Impacted Files	Coverage Δ
source/tasks/Generate_Conceptual_Help.build.ps1	91% <100%> (-3%)	:arrow_down:
source/tasks/Generate_Wiki_Content.build.ps1	88% <100%> (-4%)	:arrow_down:
source/tasks/Publish_GitHub_Wiki_Content.build.ps1	96% <100%> (-1%)	:arrow_down:

[johlju]

Merging this, it part of PR #117.