Verify downloads with astral-sh/versions checksums (#1033)

`setup-uv` currently ignores the `sha256` supplied by the default
`astral-sh/versions` manifest when a selected artifact is newer than its
bundled checksum table, allowing that download to proceed without
validation. Use the manifest checksum as a fallback after explicit and
bundled checksums, and reject manifest entries that do not provide one.
This preserves the stronger pinned hashes for known releases while
verifying newer releases without requiring an action update. Part of
#1032.

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>
Co-authored-by: William Woodruff <william@yossarian.net>
Co-authored-by: Kevin Stillhammer <kevin.stillhammer@gmail.com>

__tests__/download/download-version.test.ts

@@ -227,10 +227,10 @@ describe("download-version", () => {
       expect(mockValidateChecksum).not.toHaveBeenCalled();
     });
 
-    it("uses built-in checksums for default manifest downloads", async () => {
+    it("uses the default manifest checksum as a fallback", async () => {
       mockGetArtifact.mockResolvedValue({
         archiveFormat: "tar.gz",
-        checksum: "manifest-checksum-that-should-be-ignored",
+        checksum: "manifest-checksum",
         downloadUrl: "https://example.com/uv.tar.gz",
       });
 
@@ -248,6 +248,7 @@ describe("download-version", () => {
         "x86_64",
         "unknown-linux-gnu",
         "0.9.26",
+        "manifest-checksum",
       );
     });
 
@@ -400,6 +401,7 @@ describe("download-version", () => {
         "x86_64",
         "unknown-linux-gnu",
         "0.9.26",
+        "manifest-checksum",
       );
     });
 
@@ -425,6 +427,7 @@ describe("download-version", () => {
         "x86_64",
         "unknown-linux-gnu",
         "0.9.26",
+        "manifest-checksum",
       );
     });