bugfix: enables CSS minification via environment variable

[mattoberle]

Setting CLAYCLI_COMPILE_MINIFED (or any of the variants) did not
minify CSS or Fonts. This was due to our use of a package called
gulp-if, which has a usage like this:

gulpIf(condition, trueChild, falseChild, minmatchOptions);

Where condition is expected to either be:

1. A boolean.
2. A function that will return a boolean.

When setting CLAYCLI_COMPILE_MINIFIED from an environment variable we
end up with a string value for minify, which causes gulpIf to
evaluate to false, skipping minification.

This commit simply casts minify to a bool when calling gulpIf.

A more robust approach would be to fix typing across the board for
environment variable defaults, providing some sort of strToBool
method to cast ['1', 't', 'true', 'y', 'yes'] to true and
everything else to false.

Taking the more robust approach would require a larger refactor and is
a good follow-up candidate for this bug fix.

⚠️ Note: There is not an existing test suite to validate this
change, all tests were performed manually by running clay compile on
an existing clay project with export CLAYCLI_COMPILE_MINIFIED=true
before the commit & then checking the format of the CSS output files
(they were not minified).

After adding the commit, re-running with CLAYCLI_COMPILE_MINIFIED=true
resulted in minified CSS output.