Because common.sh is sourced before git is installed, function get_build_branch() works incorrectly by falling back to `main` which can break presubmits on release-4.Y branches.

Notice empty result of: `local -r cur_branch="$(git branch --show-current 2>/dev/null)"`

+++ 13:18:47.246463813 microshift/test/bin/common.sh:186 	get_build_branch
++++ 13:18:47.248178026 microshift/test/bin/common.sh:158 	awk '{print $NF}'
++++ 13:18:47.248319570 microshift/test/bin/common.sh:158 	awk -F. '{print $1"."$2}'
+++++ 13:18:47.248464529 microshift/test/bin/common.sh:158 	uname -m
++++ 13:18:47.250387050 microshift/test/bin/common.sh:158 	grep '^OCP_VERSION' /home/ec2-user/microshift/Makefile.version.x86_64.var
+++ 13:18:47.253051431 microshift/test/bin/common.sh:158 	local -r ocp_ver=4.15
++++ 13:18:47.254395656 microshift/test/bin/common.sh:159 	git branch --show-current
+++ 13:18:47.256099263 microshift/test/bin/common.sh:159 	local -r cur_branch=
++++ 13:18:47.257465453 microshift/test/bin/common.sh:162 	git rev-parse main
+++ 13:18:47.259158102 microshift/test/bin/common.sh:162 	local -r main_top=
++++ 13:18:47.260534195 microshift/test/bin/common.sh:163 	git merge-base '' main
+++ 13:18:47.262208582 microshift/test/bin/common.sh:163 	local -r main_base=
+++ 13:18:47.263269483 microshift/test/bin/common.sh:164 	'[' '' = '' ']'
+++ 13:18:47.264378259 microshift/test/bin/common.sh:165 	echo main
+++ 13:18:47.265496822 microshift/test/bin/common.sh:166 	return
++ 13:18:47.266732813 microshift/test/bin/common.sh:186 	SCENARIO_BUILD_BRANCH=main

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