scripts: puncover: Fix compatibility with v0.6.0

Signed-off-by: Roy Jamil <roy.jamil@ac6.fr>

Puncover v0.6.0 renamed the command-line argument
`--gcc_tools_base` to `--gcc-tools-base`.

Fixes compatibility error when running `west build -t puncover`
with puncover v0.6.0 or newer.