Track SBCL 2.6.9 source archive identity

[conornash]

Thank you so much for the kind reply on #94! As suggested, here are the others, starting with this one.

Homebrew currently ships SBCL 2.6.9, which isn't listed in sbcl-source-releases.sha256 yet, so on my Mac bootstrap carried on without implementation source and three tests that read SBCL's own source failed. This adds the 2.6.9 source hash, which I checked against the signed release checksums. I completely understand if you'd rather add new SBCL releases yourself, in which case please feel free to close this one.

The below is the agent explanation of what this PR does:

Homebrew ships SBCL 2.6.9, which sbcl-source-releases.sha256 did not list, so bootstrap skipped installing implementation source and test-self-tools, test-lisp-worker-protocol and test-lisp-busy-worker-operations failed with "Matching SBCL source is unavailable".

The added SHA-256 is for sbcl-2.6.9-source.tar.bz2. SBCL publishes checksums of the uncompressed .tar in sbcl-2.6.9-crhodes.asc, signed by Christophe Rhodes (key D683 9CA0 A67F 74D9 DFB7 0922 EBD5 95A9 100D 63CD; good signature). The downloaded .tar.bz2 decompresses to exactly the signed .tar hash (14ea1b03...78ab). The same key signs the 2.6.6 checksums that match the existing 2.6.6 pin.