ci: Upgrade Linux 18.04 -> 20.04 and DOS line endings.

[jpivarski]

This doesn't actually change the whole file, just the

awkward/.github/workflows/build-test.yml

Line 154 in c045715

runs-on: ubuntu-18.04

line. What changes the whole file is the DOS → Unix line endings.

Addresses scheduled brown-outs of Ubuntu 18.04:

[jpivarski]

Ignore the man behind the curtain. This is the only significant change.

[codecov]

Codecov Report

Merging #1616 (b812994) into main (9e17f29) will decrease coverage by 0.02%.
The diff coverage is 62.21%.

Impacted Files	Coverage Δ
src/awkward/_v2/_connect/cuda/__init__.py	0.00% <0.00%> (ø)
src/awkward/_v2/_connect/numexpr.py	88.40% <0.00%> (ø)
src/awkward/_v2/_connect/pyarrow.py	88.46% <0.00%> (ø)
src/awkward/_v2/numba.py	93.47% <0.00%> (ø)
src/awkward/_v2/operations/ak_from_avro_file.py	66.66% <0.00%> (ø)
src/awkward/_v2/operations/ak_local_index.py	100.00% <ø> (ø)
src/awkward/_v2/operations/ak_type.py	53.84% <0.00%> (ø)
src/awkward/_v2/operations/ak_transform.py	8.62% <8.62%> (ø)
src/awkward/_v2/contents/unionarray.py	86.16% <50.00%> (-0.11%)	⬇️
src/awkward/_v2/contents/bitmaskedarray.py	71.53% <53.84%> (+5.11%)	⬆️
... and 27 more