Use $PWD instead of $(pwd)

[gaelicWizard]

NOTE: This PR assumes that my basenamed branch has already been merged (#1926)! This PR does not do anything with basename/dirname; it's just based off that branch.

Description

Bash maintains the $PWD shell parameter with the current and correct full path to the present working directory. The shell builtin pwd returns the value of $PWD, so just skip the subshell and use the parameter directly.

Motivation and Context

This is part of a patch set to reduce the use of subshells.

How Has This Been Tested?

Tested locally, and all tests pass.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
  ...more of an optimization.

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[gaelicWizard]

This draft is actually ready to review assuming that my basename PR (#1926) is accepted. Anyone feel like taking a look before the other is merged? 😃

[gaelicWizard]

I'm marking this ready-for-review, but do please note that it looks larger than it is since this PR is based on the same basenamed (#1926) branch.

[NoahGorny]

lgtm, after merging #1926 of course 😄