Doesn't support target-specific export environment variable

```Makefile
export foo0=bar0
a: foo1=bar1
a: export foo2=bar2
a:
echo ${foo0} ${foo1} ${foo2}
sh -c 'echo $${foo0} $${foo1} $${foo2}'
```
```sh
$ make
echo bar0 bar1 bar2
bar0 bar1 bar2
sh -c 'echo ${foo0} ${foo1} ${foo2}'
bar0 bar2
```
```sh
$ ckati
echo bar0 bar1
bar0 bar1
sh -c 'echo ${foo0} ${foo1} ${foo2}'
bar0
```
https://www.gnu.org/software/make/manual/html_node/Target_002dspecific.html