Fix pkcs11 uri checking for key files.

[martxel]

I have found this issue when trying to use the new PKCS#11 provider support in curl.

I was getting the following error when trying to use a pkcs11 uri as a key, in curl 8.12.1:

* crypto provider not set, cannot load private key
* closing connection #0
curl: (58) crypto provider not set, cannot load private key

After checking the code that prints the error, I have found out that it is incorrectly checking if cert_file is a pkcs11 uri, instead of checking key_file:

https://github.com/curl/curl/blob/curl-8_12_1/lib/vtls/openssl.c#L1678

Probably a copy-paste error.

[testclutch]

Analysis of PR #16591 at eb2ec28a:

Test 312 failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Generated by Testclutch

[Jakuje]

Thank you for the fixup! You are right. This was copy&paste error. I miseed this as I tested with both key and cert in pkcs11. This will surface only if the cert is in file and key in pkcs11 provider.

[martxel]

Yes, I found this error because in my scenario I'm using a key in the TPM, and a certificate in a file.

Thanks to you for adding support por PKCS#11 provider, you have saved me quite a bit of work.

[bagder]

Thanks!