Fix reporting of base XDR git version (was reporting "next")

[graydon]

In this change @leighmcculloch changed the meaning of one of the version fields from rs-stellar-xdr. As a result, lately we've just been reporting the string "next" for the base XDR git version, rather than a proper hash.

This PR adapts to the change and thus fixes the externally observable behaviour back to what it was before.

Before this change:

$ ./src/stellar-core version
v19.8.0-33-g53ea43ace
rust version: rustc 1.68.0 (2c8cc3432 2023-03-06)
soroban-env-host: 
    package version: 0.0.14
    git version: 7dde7cb0b35957856a1e0c11432e00532ae20cf8
    interface version: 32
    rs-stellar-xdr:
        package version: 0.0.14
        git version: 07aff1190dc3f97b889d31b2bfb77098e7db55ce
        base XDR git version: next

With this change:

$ ./src/stellar-core version
v19.8.0-33-g53ea43ace-dirty
rust version: rustc 1.68.0 (2c8cc3432 2023-03-06)
soroban-env-host: 
    package version: 0.0.14
    git version: 7dde7cb0b35957856a1e0c11432e00532ae20cf8
    interface version: 32
    rs-stellar-xdr:
        package version: 0.0.14
        git version: 07aff1190dc3f97b889d31b2bfb77098e7db55ce
        base XDR git version: 7356dc237ee0db5626561c129fb3fa4beaabbac6

[sisuresh]

r+ f404b16