Prefer target.env values over build.env config.

We currently use `build.env` over `target.$(...).env`, which means we
cannot set a default value and then override it for a specific target.
Given the following config file:

```toml
[build.env]
xargo = true

[target.aarch64-unknown-linux-gnu.env]
xargo = false
```

We currently would use `xargo` even on `aarch64-unknown-linux-gnu`, when
we should not, and only use `xargo` for every other target.

Closes #773.

Author: Alexhuszagh

src/config.rs

@@ -137,13 +137,13 @@ impl Config {
             (None, None)
         };
 
-        match (env_build, toml_build) {
+        match (env_target, toml_target) {
             (Some(value), _) => return Some(value),
             (None, Some(value)) => return Some(value),
             (None, None) => {}
         };
 
-        match (env_target, toml_target) {
+        match (env_build, toml_build) {
             (Some(value), _) => return Some(value),
             (None, Some(value)) => return Some(value),
             (None, None) => {}