ref(builds-cacher): gate .git on client-provided unpacker

`.git` was pushed unconditionally into getSortedFormats's candidate
ext list, while sibling unpacker formats (.tar.xz, .tar.zst, .zip,
.7z) are gated on whether the caller's `formats` argument signals
the client has the corresponding tool.

Make `.git` consistent: only add it to the candidate list when
formats includes 'git'. The default WEBI_FORMATS ('tar,exe,zip,xz,
dmg') doesn't include git, so the change is a no-op for the
current default. Clients that want git-source packages installed
can pass `?formats=tar,exe,zip,xz,dmg,git` (or set the equivalent
in a future client-side probe).

For packages that have only a git-source asset (e.g. some vim
plugins), the existing fallback to `packages[0]` still returns the
git entry — behavior unchanged. The only observable change is for
packages where both a binary and a git fallback exist for the same
triplet: previously the git entry could win over the binary; now
it wins only when the client opts in.

_webi/builds-cacher.js

@@ -557,7 +557,10 @@ BuildsCacher.create = function ({ ALL_TERMS, installers, caches }) {
       exts.push('.gz');
       exts.push('.sh');
     }
-    exts.push('.git');
+    let hasGit = formats.includes('git') || formats.includes('.git');
+    if (hasGit) {
+      exts.push('.git');
+    }
 
     // Fallbacks
     // (we include everything to bubble an extract error over not found)