build-gnu.sh: improve setup instructions

[cakebaker]

This PR improves the instructions to setup the GNU coreutils.

Before the changes:

$ ./util/build-gnu.sh 
UU_MAKE_PROFILE='debug'
Could not find GNU coreutils (expected at '/home/dho/projects/gnu')
Run the following to download into the expected path:
git clone --recurse-submodules https://github.com/coreutils/coreutils.git "/home/dho/projects/gnu"
After downloading GNU coreutils to "/home/dho/projects/gnu" run the following commands to checkout latest release tag
cd "/home/dho/projects/gnu"
git fetch --all --tags
git checkout tags/v9.8

After the changes:

$ ./util/build-gnu.sh 
UU_MAKE_PROFILE='debug'
Could not find the GNU coreutils (expected at '/home/dho/projects/gnu')
Download them to the expected path:
  git clone --recurse-submodules https://github.com/coreutils/coreutils.git "/home/dho/projects/gnu"
Afterwards, checkout the latest release tag:
  cd "/home/dho/projects/gnu"
  git fetch --all --tags
  git checkout tags/v9.8

[codspeed-hq]

CodSpeed Performance Report

Merging #8731 will not alter performance

_{Comparing cakebaker:build_gnu_improve_instructions (d6da7e0) with main (a27a124)}

Summary

✅ 45 untouched
⏩ 1 skipped¹

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)