fix: specify correct path for tools/go.mod when running benchmark

[kyungeunni]

Motivation/summary

Benchmark fails with the following error message:

go: open tools/go.mod: no such file or directory
cat: write error: Broken pipe

This is caused by specifying the wrong path for the target file; the working directory is apm-server/testing/apmbenech and the target file is in apm-server/tools/go.mod

make[1]: Entering directory '/home/runner/work/apm-server/apm-server/testing/benchmark'

fix it by specifying the correct path for the tools/go.mod file by using GITROOT:

apm-server/go.mk

Line 6 in ab4b728

GITROOT ?= $(shell git rev-parse --show-toplevel)

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Related issues

Introduced by https://github.com/elastic/apm-server/pull/13421/files

[mergify]

This pull request does not have a backport label. Could you fix it @kyungeunni? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

NOTE: backport-skip has been added to this pull request.