getproviders: account for occasionally missing Host header in errors

[radeksimko]

This is a follow-up PR to #30810 and #29298

They each aimed to add more context to the following error messages:

Error while installing citrix/citrixadc v1.13.0: could not query provider
registry for registry.terraform.io/citrix/citrixadc: failed to retrieve
authentication checksums for provider: 403 Forbidden

Error: Failed to install provider
Error while installing 1password/onepassword v1.1.2: could not query provider
registry for registry.terraform.io/1password/onepassword: failed to retrieve
authentication checksums for provider: the request failed after 2 attempts,
please try again later: 500

Based on some recent CI runs, it became clear that Request.Host may sometimes be empty, which results in the following error message:

Error: Failed to install provider

Error while installing assistanz/stackbill v0.1.0: could not query provider
registry for registry.terraform.io/assistanz/stackbill: failed to retrieve
cryptographic signature for provider: 403 Forbidden returned from 

See https://github.com/hashicorp/terraform-ls/runs/7574713438?check_suite_focus=true#step:6:46

I don't actually know under what exact circumstances would the Host be missing. It is clear however from reading the implementation of net/http.Request.Write() that this may happen.

I therefore propose to mimic what the stdlib does here, to ensure we always expose the host in the error message.

I expect #31524 can be updated to take advantage of this too, once this PR is merged.

[kmoe]

LGTM

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