EC2 Auth Fails on Some Instances

[maths22]

Describe the bug
EC2 Vault login fails with failed to parse the BER encoded PKCS#7 signature: ber2der: Invalid BER format for some instances

To Reproduce
Steps to reproduce the behavior:

1. Run vault write -format=json "auth/aws/login" role="$role" pkcs7="$pkcs7" nonce="$nonce"
2. Get the error from above

This only happens on a subset of ec2 instances (those that include a certificate in the response from http://169.254.169.254/latest/dynamic/instance-identity/pkcs7) We could not find a pattern as to which instances did and didn't include the certificate, but the bug reliably happens if the certificate is included.

Expected behavior
It should not break

Environment:

• Vault Server Version (retrieve with vault status): 1.8.2
• Vault CLI Version (retrieve with vault version): 1.6.2
• Server Operating System/Architecture: linux/amd64

Additional context
When #12340 / #12361 were pulled in, they happened before this upstream PR: mozilla-services/pkcs7#61

Without the upstream pr, certain valid BER files are rejected as invalid and are mis-parsed.