[main] Makefile: set default branch when initializing git

[thaJeztah]

This should suppress these hints printed during build:

hint: Using 'master' as the name for the initial branch. This default branch name
hint: is subject to change. To configure the initial branch name to use in all
hint: of your new repositories, which will suppress this warning, call:
hint:
hint: 	git config --global init.defaultBranch <name>
hint:
hint: Names commonly chosen instead of 'master' are 'main', 'trunk' and
hint: 'development'. The just-created branch can be renamed via this command:
hint:
hint: 	git branch -m <name>

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

Looks like the git version of the Jenkins agents is too old (probably would work once they're updated to a newer LTS version);

+ make CREATE_ARCHIVE=1 docker.io/balenalib/rpi-raspbian:bookworm
make BUILD_IMAGE="docker.io/balenalib/rpi-raspbian:bookworm" build
make[1]: Entering directory '/home/ubuntu/workspace/containerd-packaging_PR-366'
git init --initial-branch=main src/github.com/containerd/containerd
error: unknown option `initial-branch=main'
usage: git init [-q | --quiet] [--bare] [--template=<template-directory>] [--shared[=<permissions>]] [<directory>]

    --template <template-directory>
                          directory from which templates will be used
    --bare                create a bare repository
    --shared[=<permissions>]
                          specify that the git repository is to be shared amongst several users
    -q, --quiet           be quiet
    --separate-git-dir <gitdir>
                          separate git dir from working tree

make[1]: *** [Makefile:60: src/github.com/containerd/containerd] Error 129
make[1]: Leaving directory '/home/ubuntu/workspace/containerd-packaging_PR-366'
make: *** [Makefile:72: docker.io/balenalib/rpi-raspbian:bookworm] Error 2
script returned exit code 2

[thaJeztah]

Yup; looks like ubuntu 20.04 has git

git --version
git version 2.25.1

Which doesn't have the option;

NAME
       git-init - Create an empty Git repository or reinitialize an existing one

SYNOPSIS
       git init [-q | --quiet] [--bare] [--template=<template_directory>]
                 [--separate-git-dir <git dir>]
                 [--shared[=<permissions>]] [directory]

[thaJeztah]

on ubuntu 22.04 it's there; I can check if we have agents with 22.04

git --version
git version 2.34.1

NAME
       git-init - Create an empty Git repository or reinitialize an existing one

SYNOPSIS
       git init [-q | --quiet] [--bare] [--template=<template_directory>]
                 [--separate-git-dir <git dir>] [--object-format=<format>]
                 [-b <branch-name> | --initial-branch=<branch-name>]
                 [--shared[=<permissions>]] [directory]

[thaJeztah]

• opened [main] Jenkinsfile: update to use Ubuntu 22.04 agents #369