Close manifest after writing it so it can be removed later in code

[roffe]

Fixes a bug where the fyne package command would fail on Windows with error:

failed to remove manifest: remove C:\Users\roffe\go\src\github.com\roffe\t7logger\t7logger.exe.manifest: The process cannot access the file because it is being used by another process.

[coveralls]

Coverage: 61.982% (-0.002%) from 61.984% when pulling 461258a on roffe:patch-2 into bc2b858 on fyne-io:develop.

[Jacalz]

Thanks for the PR. I left a comment in-line about better placement for the close.

[Jacalz]

Thanks for the fix and congratulations on landing your first PR to Fyne :)

I wish that the static analysis would catch unclosed readers and writers. I suspect there might be more places where we are missing close calls.