Auto merge of rust-lang#7874 - ehuss:update-tar, r=Eh2406

Update tar.

Updates to the latest tar.  rust-lang/rust is currently on 0.4.20.  The change I'm most interested in is fixing the TarError cause/source, so that Cargo reports a better error message.  Compare:

```
Caused by:
  failed to unpack `/…/registry/src/github.com-1ecc6299db9ec823/curl-sys-0.4.25/curl/docs/libcurl/libcurl-multi.3`
```

to the new version:

```
Caused by:
  failed to unpack `curl-sys-0.4.25/curl/docs/libcurl/libcurl-multi.3` into `/…/registry/src/github.com-1ecc6299db9ec823/curl-sys-0.4.25/curl/docs/libcurl/libcurl-multi.3`

Caused by:
  No space left on device (os error 28)
```

Cargo.toml

@@ -60,7 +60,7 @@ serde_ignored = "0.1.0"
 serde_json = { version = "1.0.30", features = ["raw_value"] }
 shell-escape = "0.1.4"
 strip-ansi-escapes = "0.1.0"
-tar = { version = "0.4.18", default-features = false }
+tar = { version = "0.4.26", default-features = false }
 tempfile = "3.0"
 termcolor = "1.0"
 toml = "0.5.3"