action fails to process non-ASCII Go source file on Windows

[maruel]

Problem: The action when running on Windows fails to set the default codepage to UTF-8, it likely runs in the default one CP-1252.

Example of source files containing non-ASCII characters like µ and ¹²:
https://github.com/periph/conn/blob/main/physic/doc.go
https://github.com/periph/conn/blob/main/physic/units.go

Windows (fails to handle utf-8):
https://github.com/periph/conn/actions/runs/10322595134/job/28578153166

(...)
info - 2024-08-09 16:37:33,312 -- Found 1 coverage files to report
info - 2024-08-09 16:37:33,312 -- > D:\a\conn\conn\coverage.txt
warning - 2024-08-09 16:37:33,359 -- There was an issue decoding: physic/doc.go, file fixes were not applied to this file.
warning - 2024-08-09 16:37:33,359 -- There was an issue decoding: physic/units.go, file fixes were not applied to this file.
info - 2024-08-09 16:37:34,203 -- Your upload is now processing. When finished, results will be available at: https://app.codecov.io/github/periph/conn/commit/dd522d694dd7eba02[46](https://github.com/periph/conn/actions/runs/10322595134/job/28578153166#step:7:47)e8a248afdac95017f6679
info - 2024-08-09 16:37:34,516 -- Process Upload complete

macOS (handles utf-8): https://github.com/periph/conn/actions/runs/10322595134/job/28578152736

(...)
info - 2024-08-09 16:35:42,246 -- Found 1 coverage files to report
info - 2024-08-09 16:35:42,246 -- > /Users/runner/work/conn/conn/coverage.txt
info - 2024-08-09 16:35:42,635 -- Your upload is now processing. When finished, results will be available at: https://app.codecov.io/github/periph/conn/commit/dd522d694dd7eba0246e8a248afdac95017f6679
info - 2024-08-09 16:35:42,885 -- Process Upload complete