[ci] reduce repetition of BUILD_DIRECTORY in CI scripts

[jameslamb]

For a long time, this project's CI scripts have used mostly the following combinations:

• lots of absolute paths to source files
• repetitive uses of cd to reset the working directory

With the changes in #6266, I believe that these patterns are unnecessary and that builds will fail if instead these scripts use relative paths that end up pointing to files that don't exist.

In the interest of making the CI configuration easier for contributors to read and understand, this proposes:

• removing unnecessary cd calls
• replacing most uses of absolute paths with relative paths

[borchero]

Nice!

[jameslamb]

Thanks as always for all the reviews @borchero !