ci: twister: Add `--specs` to ccache ignore option list

This commit adds the compiler `--specs=*` flag to the ccache ignore option
list because ccache is unable to resolve the toolchain-provided specs file
path and will consider source files to be uncacheable if it is unable to
read the specified specs file.

Note that adding `--specs=*` to the ignore option list is not a problem
because it is unlikely for the content of the toolchain libc spec file to
change without the compiler executable itself changing.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>