bootstd: Fix a memory leak in the efi manager bootflow

efi_get_var() allocates memory which has to be freed after the value of
the variable is consumed. Free the memory properly

Fixes: f2bfa0cb ("bootstd: Make efi_mgr bootmeth work for non-sandbox setups")
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>