Parameterize run_tests.sh go executable with GO env var
Parameterize run_tests.sh go executable with GO env var

Default to 'go' to retain existing behavior.

This more easily allows to run all tests with other supported or
upcoming Go versions, e.g.

$ GO=go1.16.10 ./run_tests.sh

or

$ GO=gotip ./run_tests.sh

decred/dcrwallet,decred/dcrwallet