mkosi: Enable InstallDirectory and SourceFileTransferFinal options

- InstallDirectory caches the install part of the build process
which speeds up incremental builds a little and allows inspecting
the installed components in mkosi.installdir.
- SourceFileTransferFinal copies the source files to the final
image which makes the gdb experience in qemu/systemd-nspawn a bit
nicer as it can now find the source files and show the source code
in the gdb cli itself.

mkosi.default.d/10-systemd.conf

@@ -10,6 +10,8 @@ HostonlyInitrd=yes
 [Packages]
 BuildDirectory=mkosi.builddir
 Cache=mkosi.cache
+InstallDirectory=mkosi.installdir
+SourceFileTransferFinal=copy-git-others
 
 [Host]
 QemuHeadless=yes

src/basic/stat-util.c

@@ -226,13 +226,12 @@ int fd_is_network_fs(int fd) {
 }
 
 int path_is_temporary_fs(const char *path) {
-        _cleanup_close_ int fd = -1;
+        struct statfs s;
 
-        fd = open(path, O_RDONLY|O_CLOEXEC|O_NOCTTY|O_PATH);
-        if (fd < 0)
+        if (statfs(path, &s) < 0)
                 return -errno;
 
-        return fd_is_temporary_fs(fd);
+        return is_temporary_fs(&s);
 }
 
 int stat_verify_regular(const struct stat *st) {