common.sh: Handle single quotes in load_environment_var()

This is used to read version.txt, which was written with single quotes
when Jenkins reverted a build. These quotes were then erroneously
written into /etc/os-release, which caused havoc. This has already been
addressed in jenkins-os, but let's make this code more robust anyway.

Fix this by grepping out the lines we need (in one go) and actually
parsing them with Bash. Less safe, but we already parse version.txt with
Bash in other places.

Signed-off-by: James Le Cuirot <jlecuirot@microsoft.com>

common.sh

@@ -296,12 +296,10 @@ load_environment_allowlist() {
 }
 
 load_environment_var() {
-  local file="$1" name value
-  shift
-  for name in "$@"; do
-    value=$(grep "^${name}=" "${file}" | sed 's|"||g')
-    [[ -n "${value}" ]] && export "${value}"
-  done
+  local file="$1"; shift
+  unset "${@}"
+  . <(grep -f <(printf "^%s=\n" "${@}") "${file}")
+  export "${@}"
 }
 
 # Find root of source tree