bump actions/attest from 2.3.0 to 2.4.0 (#178)

Signed-off-by: Brian DeHamer <bdehamer@github.com>

README.md

@@ -127,6 +127,10 @@ Attestations are saved in the JSON-serialized [Sigstore bundle][8] format.
 If multiple subjects are being attested at the same time, a single attestation
 will be created with references to each of the supplied subjects.
 
+The absolute path to the generated attestation is appended to the file
+`${RUNNER_TEMP}/created_attestation_paths.txt`. This file will accumulate the
+paths to all attestations created over the course of a single workflow.
+
 ## Attestation Limits
 
 ### Subject Limits

action.yml

@@ -71,7 +71,7 @@ runs:
       id: generate-sbom-predicate
       with:
         sbom-path: ${{ inputs.sbom-path }}
-    - uses: actions/attest@v2.3.0
+    - uses: actions/attest@ce27ba3b4a9a139d9a20a4a07d69fabb52f1e5bc # v2.4.0
       id: attest
       with:
         subject-path: ${{ inputs.subject-path }}