switch to SecTrustEvaulateWihError to avoid deprecation warning

[ropnop]

Hi there! Building smimesign or certstore currently displays a deprecation warning (#85):

# github.com/github/smimesign/certstore
cgo-gcc-prolog:473:11: warning: 'SecTrustEvaluate' is deprecated: first deprecated in macOS 10.15 [-Wdeprecated-declarations]
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Security.framework/Headers/SecTrust.h:353:10: note: 'SecTrustEvaluate' has been explicitly marked deprecated here

I dove into this and this is what I came up with.

SecTrustEvaluate has been marked as deprecated and should be replaced by SecTrustEvaluateWithError according to https://developer.apple.com/documentation/security/1394363-sectrustevaluate

The new function, SecTrustEvaluateWithError returns a bool if the certificate is trusted or not, which I don't believe we care about or need here. It optionally takes a reference to a CFError as well, in which I pass in nil (but we could capture/parse it if we think it's necessary). According to the docs, to make a determination if the failure is recoverable, the docs say to make a call to SecTrustGetTrustResult and evaluate that OSStatus result.

I've modified the code to call SecTrustEvaluateWithError followed by a call to SecTrustGetTrustResult and detemine if that error is recoverable or not. The tests are all passing and I haven't noticed any side effects other than the deprecation warning being gone :)

Let me know if you think this is an acceptable PR!