os: don't repeat dir argument in CreateTemp error
os: don't repeat dir argument in CreateTemp error

The dir argument is already in prefix, we shouldn't add it again.

Change-Id: I42a158bec3a43950fce24f57b808da3ad8c5ef5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/383636
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

nawawi/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,golang/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,golang/go,golang/go,cloudflare/go,CAFxX/go