cannot install certs on Debian

[ju1ius]

Hi,

Installing a certificate into to the system trust store is broken on Debian Sid (see also: smallstep/cli#1043).

$ cat /etc/debian_version
trixie/sid
$ ./bin/truststore my_cert.crt
install is not supported on this system

On Debian the update-ca-certificates binary is located under /usr/sbin, which is not in the path of non-root users.

This causes the exec.LookPath call in truststore_linux.go's init function to always fail and setting SystemTrustCommand to nil, which in turn causes installPlatform to incorrectly return ErrNotSupported.

Since the command (and the equivalent step certificate install and step ca bootstrap --install) should not be run as root, the exec.LookPath check should be removed from the init function, letting commandWithSudo(SystemTrustCommand...) fail (with an actually useful message) if SystemTrustCommand is not found, which is what mkcert does.