ci: windows

[mmcloughlin]

Fix tests on Windows.

There are some annoyances possibly related to line-endings, for example:

--- FAIL: TestFormatGolden (0.02s)
    --- FAIL: TestFormatGolden/poly1305 (0.00s)
Error:         golden_test.go:[7](https://github.com/mmcloughlin/mathfmt/actions/runs/5986804869/job/16240334229?pr=24#step:5:8)2: line number mismatch: got 30[8](https://github.com/mmcloughlin/mathfmt/actions/runs/5986804869/job/16240334229?pr=24#step:5:9) expect 307
    --- FAIL: TestFormatGolden/stats (0.00s)
Error:         golden_test.go:72: line 5:
            	got    = "//\t\t∑ xᵢ / N"
            	expect = "//\t∑ xᵢ / N"
Error:         golden_test.go:72: line 1[9](https://github.com/mmcloughlin/mathfmt/actions/runs/5986804869/job/16240334229?pr=24#step:5:10):
            	got    = "//\t\tσ² = ∑ (xᵢ - μ)² / N"
            	expect = "//\tσ² = ∑ (xᵢ - μ)² / N"
FAIL
exit status 1
FAIL	github.com/mmcloughlin/mathfmt	0.061s
Error: Process completed with exit code 1.

https://github.com/mmcloughlin/mathfmt/actions/runs/5986804869/job/16240334229?pr=24#step:5:7