doc/examples: Actually use suite variable
doc/examples: Actually use suite variable

We declared the suite variable (defaulting to stretch), but only used it
for the filename and not for building the OS, which took a hardcoded
suite name.

Use the suite variable in OS building, changing the default to buster to
match the previous hard-coding.

Signed-off-by: Daniel Stone <62e7e355d5d8400fa8384448796af0bf6be4ffae@collabora.com>

go-debos/debos