certbot generates a new private key every time it obtains a new certificate.
If you want to pin the end-entity certificate you’ll have to generate your own private key and CSR and use certbot --csr to obtain your certificate (which also requires you to set up renewal manually on your own).
Alternatively, you could instead pin the active and backup intermediate certificates (Let’s Encrypt Authority X3 and Let’s Encrypt Authority X4) or the current and future root certificates (DST Root CA X3 and ISRG Root X1).