script: use tools.mod for tools install

[mmcloughlin]

The stress test CI job has been failing due to an error in bootstrap:

/~https://github.com/mmcloughlin/addchain/runs/2129921772?check_suite_focus=true#step:5:24

The problem relates to gobin under Go 1.16 myitcv/gobin#103.

This PR fixes the issue by using a tools.mod file to manage tools dependencies as suggested in:

https://www.bwplotka.dev/2020/bingo/
/~https://github.com/prometheus-operator/prometheus-operator/blob/5a2412883718ffe889fe741d5e0e5e7ae6b5f183/scripts/go.mod

[codecov-io]

Codecov Report

Merging #61 (9f09bf6) into master (6fcb85f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #61   +/-   ##
=======================================
  Coverage   76.32%   76.32%           
=======================================
  Files          25       25           
  Lines         997      997           
=======================================
  Hits          761      761           
  Misses        189      189           
  Partials       47       47           

Flag	Coverage Δ
unittests	76.32% <ø> (ø)

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