fix: building twice was broken

[henryiii]

Editable installs still don't work, but at least this allows building more than once.

[codecov]

Codecov Report

Merging #1254 (faccafe) into main (ab65e20) will increase coverage by 0.23%.
The diff coverage is 60.48%.

Impacted Files	Coverage Δ
src/awkward/_v2/__init__.py	100.00% <ø> (ø)
src/awkward/_v2/_connect/numpy.py	58.63% <0.00%> (+0.17%)	⬆️
src/awkward/_v2/behaviors/string.py	71.42% <ø> (+16.88%)	⬆️
src/awkward/_v2/contents/__init__.py	100.00% <ø> (ø)
src/awkward/_v2/contents/bitmaskedarray.py	64.31% <0.00%> (-0.14%)	⬇️
src/awkward/_v2/contents/bytemaskedarray.py	82.76% <0.00%> (-0.05%)	⬇️
src/awkward/_v2/contents/emptyarray.py	69.36% <0.00%> (-0.18%)	⬇️
src/awkward/_v2/contents/indexedarray.py	60.64% <0.00%> (-0.10%)	⬇️
src/awkward/_v2/contents/indexedoptionarray.py	85.40% <0.00%> (+0.15%)	⬆️
src/awkward/_v2/contents/listarray.py	87.95% <0.00%> (+0.21%)	⬆️
... and 143 more