chore: set initial branch name for bogo checkout.

Prior to this commit the `fetch-and-build` script with newer `git`
versions would spit out an ANSI coloured warning about choosing an
initial branch name for the bogo test-suite checkout.

This commit simply specifies the `--initial-branch` to be `main` to
silence the unnecessary output.

bogo/fetch-and-build

@@ -5,7 +5,7 @@ set -ex
 mkdir -p bogo
 pushd bogo
 
-git init
+git init --initial-branch main
 git config core.sparsecheckout 1
 cat << EOF > .git/info/sparse-checkout
 go.mod