spawn: set encoding utf8

so we get pretty error messages

add

```js
encoding: "utf8",
```

to

https://github.com/ds300/patch-package/blob/5c2c92bf504885fba4840870a23fc8999c00e572/src/spawnSafe.ts#L10-L13

currently we get binary output, for example

```
patch-package jest-snapshot
patch-package 6.5.0
• Creating temporary folder
• Installing jest-snapshot@26.6.2 with yarn
warning package.json: No license field
warning No license field
error Couldn't find any versions for "jest-snapshot" that matches "26.6.2, jest-snapshot@npm:^26.6.2"

{
status: 1,
signal: null,
output: [
null,
,

],
pid: 2418758,
stdout: ,
stderr: ,
error: null
}
```