cve-check: Don't use f-strings

Since we're keeping cve-check aligned between the active branches,
and dunfell is supported on Python 3.5, we can't use f-strings.

Signed-off-by: Ernst Sjöstrand <ernstp@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>