Stop using curl's --time-cond / -z flag (#610)

We started using the -z flag with curl to avoid downloading an
ecosystem index if there were no updates to actually fetch. This
usually works fine, but some people have had issues where an
index was not getting updated even when explicitly using
zef update. I consider using --remote-time, but there are other
time checks that zef does on that file that expect the times of
the file to represent when zef created them (not what the server
modification time is). For now we will solve this be not using
the -z flag anymore.

Resolves #602

Author: ugexe

lib/Zef/Service/Shell/curl.rakumod

@@ -106,7 +106,7 @@ class Zef::Service::Shell::curl does Fetcher does Probeable {
             my $cwd := $save-as.parent;
             my $ENV := %*ENV;
             my $cmd := self!command();
-            my $proc = Zef::zrun-async($cmd, '--silent', '-L', '-z', $save-as.absolute, '-o', $save-as.absolute, $uri);
+            my $proc = Zef::zrun-async($cmd, '--silent', '-L', '-o', $save-as.absolute, $uri);
             $stdout.emit("Command: {$proc.command}");
             whenever $proc.stdout(:bin) { }
             whenever $proc.stderr(:bin) { }