[release-1.8 branch] CI (Buildkite): remove the .buildkite folder

[DilumAluthge]

This pull request was automatically generated using the following script:

declare -a arr=("master" "release-1.6" "release-1.7" "release-1.8")
for BASE_BRANCH in "${arr[@]}"
do
    echo "BASE_BRANCH: ${BASE_BRANCH:?}"
    cd $(mktemp -d)
    git clone git@github.com:JuliaLang/julia.git
    cd julia
    git checkout ${BASE_BRANCH:?}
    git checkout -B dpa/remove-buildkite-folder-${BASE_BRANCH:?}
    rm -rf .buildkite
    rm -rf .buildkite-external-version
    echo "main" > .buildkite-external-version
    git add -A
    git commit -m "[\`${BASE_BRANCH:?}\` branch] CI (Buildkite): remove the \`.buildkite\` folder"
    git push -f origin dpa/remove-buildkite-folder-${BASE_BRANCH:?}
    cd ..
done

[staticfloat]

@DilumAluthge why don't we have buildkite statuses here? 😅

[DilumAluthge]

I'll look into it now.

[DilumAluthge]

I have documented the necessary steps here: JuliaCI/julia-buildkite#43

[DilumAluthge]

@KristofferC FYI, this PR directly targets release-1.8. However, the only files modified by this PR are:

1. .buildkite-external-version
2. .buildkite/*
3. .gitignore

None of those files should have an effect on PkgEval, so I think it's fine to merge this PR directly into release-1.8.

[DilumAluthge]

Okay, Buildkite is now running on this PR.

@staticfloat If all Buildkite jobs pass on this PR, then it is good to merge from my point of view.

[staticfloat]

Looks like an ASAN failure. @tkf or @vtjnash is this expected on 1.8?

[DilumAluthge]

Ah that's right, @vtjnash fixed asan in #44420.

We need to backport #44420 to release-1.8 before we can merge this PR.