fix test using non-unique tempfile

Cargo.lock

@@ -1432,6 +1432,7 @@ dependencies = [
  "serde_json",
  "structopt",
  "tar",
+ "tempfile",
  "thiserror",
  "tokio 0.2.16",
  "url",

http/Cargo.toml

@@ -37,3 +37,4 @@ bytes = "0.5.4"
 [dev-dependencies]
 hex = "0.4.2"
 hex-literal = "0.2.1"
+tempfile = "3.1.0"

http/src/v0/root_files.rs

@@ -269,8 +269,8 @@ mod tests {
                     let size = header.size()?;
 
                     // writing to file is the only supported way to get the contents
-                    let mut temp_file = std::env::temp_dir();
-                    temp_file.push("temporary_file_for_testing.txt");
+                    let tempdir = tempfile::tempdir()?;
+                    let temp_file = tempdir.path().join("temporary_file_for_testing.txt");
                     entry.unpack(&temp_file)?;
 
                     let bytes = std::fs::read(&temp_file);