The `printf` formatters in our standalone fuzzing driver are currently
using the "%m" specifier, which is a GNU extension that prints the error
message for the error code in `errno`. As we're using libgit2 functions
in both cases anyway, let's just use `git_error_last` instead to make
this valid on all platforms.

Use `p_unlink` instead of `unlink` to remove the generated packfiles in
our packfile fuzzer. Like this, we do not have to worry about using
proper includes that are known on all platforms, especially Win32.

The `snprintf` function does not exist on Win32, it only has
`_snprintf_s` available. Let's just avoid any cross-platform hassle and
use our own `git_buf` functionality instead.

There's multiple headers included in our fuzzers that aren't required at
all. Furthermore, some of them are not available on Win32, causing
builds to fail. Remove them to fix this.

In order to guarantee that our fuzzers build just fine on the Windows
platform, let's enable building fuzzers on all Powershell-based builds.