Fixed wrong TX url in email notifications for mainnet

[atapin]

Fixes #7938

Motivation

Currently, email notifications on mainnet contain wrong tx address, e.g.:
https://blockscout.com/tx/0x8ae7becf6abe11ff4eec490537f26e9cac228af37dbc3503e9bc030162e42b79 instead of https://eth.blockscout.com/tx/0x8ae7becf6abe11ff4eec490537f26e9cac228af37dbc3503e9bc030162e42b79

Bug Fixes

The url-building logic contained redundant check for prod environment that led to wrong hostname substitution. According to the docs, the environment should have correct value in BLOCKSCOUT_HOST ENV variable.

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

[atapin]

To maintainers: advices on how to add a regression on module-private API are appreciated. There's already good test for Explorer.Account.Notify.Email, but it was not ready for dependency on environment-specific logic.

[vbaranov]

Nice catch, thank you! I will improve tests for this module in frame of #7954