export_strds: specify path to the directory where output is written

[metzm]

By default, t.rast.export in _export_strds() uses the current directory for raster exports which can be dangerous. In actinia, output should be written to a temporary directory in the current temporary GRASS db, as in e.g. _export_raster().

Important rule: if you don't know what the current working directory is, don't use it (esp. for large temporary files).