revert recent changes to variable assignments

[majewsky]

This reverts commit d3da286 and 8f69811.

These commits seem to have been made without understanding how the variable mechanism is supposed to work. They also invalidate the documentation that's right next to the changed lines.

[github-actions]

Merging this branch will decrease overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/sapcc/go-makefile-maker/internal/makefile	8.00% (-0.02%)	👎

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/sapcc/go-makefile-maker/internal/makefile/makefile.go	0.00% (ø)	196 (+1)	0	196 (+1)

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

[stanislav-zaprudskiy]

These commits seem to have been made without understanding how the variable mechanism is supposed to work. They also invalidate the documentation that's right next to the changed lines.

You may want to update the originally intended and documented behavior as well to reflect the current behavior -

go-makefile-maker/README.md

Lines 458 to 470 in 18ddcb2

	When invoking the respective targets utilizing the variables, it's possible to either append to the provided values by modifying the calling environment:
	```console
	$ GO_BUILDFLAGS="-work -v" make build/go-makefile-maker
	env go build -mod vendor -work -v -ldflags ...
	```
	or to override their values by passing the variables as `make` arguments:
	```console
	$ make build/go-makefile-maker GO_BUILDFLAGS="-mod=mod"
	env go build -mod=mod -ldflags ...
	```