ARROW-7733: [Developer] Download new enough Go locally in release verification script
ARROW-7733: [Developer] Download new enough Go locally in release verification script

Some developers have failed to run the Go part of the verification script because the version of Go required is newer than what is installed by some Linux package managers, such as Ubuntu 18.04

This also fixes the pesky need to write `TEST_DEFAULT=0 TEST_SOURCE=1 ... source $VERSION $RC_NUM` when `TEST_SOURCE=1` is redundant with the `source` mode. Additionally miniconda is only installed when it is needed by one of the verification steps

Closes #6710 from wesm/ARROW-7733

Authored-by: Wes McKinney <3be70eed0275ec2606f002bff3514252e145d204@apache.org>
Signed-off-by: Wes McKinney <3be70eed0275ec2606f002bff3514252e145d204@apache.org>

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