command: Fix bug when using terraform login on Windows

[alisdair]

Restructure the credentials storing code for compatibility with Windows system calls. Fixes #24388. Changes:

• We need to close the temporary file before moving it
• file.Chmod is not supported, so use os.Chmod
• os.Chmod also requires the file is closed, so move it to the end of the procedure

Note that the chmod doesn't actually do anything useful on Windows. To actually make this file user-only, we'd need to use something like go-acl. I'm open to doing that in this PR, but I thought the smallest change to fix the bug was worth looking at first.

Screenshot

Before:

After:

[codecov]

Codecov Report

Merging #24397 into master will increase coverage by 0.00%.
The diff coverage is 33.33%.

Impacted Files	Coverage Δ
command/cliconfig/credentials.go	60.49% <33.33%> (ø)
dag/marshal.go	52.27% <0.00%> (-1.14%)	⬇️
backend/remote/backend_common.go	55.63% <0.00%> (+0.70%)	⬆️

[mildwonkey]

I approve both this PR and overriding codecov's objections!

[ghost]

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