internal/dl: redirect to downloadBaseURL for GPG signatures
internal/dl: redirect to downloadBaseURL for GPG signatures

Fixes golang/go#35717

Change-Id: I6d063fb1d6c33b60a7bcf34cda832011a2fc120d
GitHub-Last-Rev: 9706ecb20d608d8688f4cf5dfea81c59ba718f6c
GitHub-Pull-Request: golang/website#6
Reviewed-on: https://go-review.googlesource.com/c/website/+/208158
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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