version: parse Void Linux version strings

We have ~2.5k nodes running Void Linux, which report a version string
like `1.96.2_1 (Void Linux)`. Previously these versions would fail to
parse, because we only expect a hyphen and `extraCommits` after the
major/minor/patch numbers.

Fix the version parsing logic to handle this case.

Updates #19148

Change-Id: Ica4f172d080af266af7f0d69bb31483a095cd199
Signed-off-by: Alex Chan <alexc@tailscale.com>

version/cmp.go

@@ -103,6 +103,11 @@ func parse(version string) (parsed, bool) {
 		}
 	}
 
+	// Ignore trailer like '_1 (Void Linux)'.
+	if rest[0] == '_' && strings.HasSuffix(rest, " (Void Linux)") {
+		return ret, true
+	}
+
 	// Optional extraCommits, if the next bit can be completely
 	// consumed as an integer.
 	if rest[0] != '-' {