age: error: failed to wrap key for recipient #0: test plugin: write |1: broken pipe

## Environment

* OS: NixOS 23.05
* age version: 1.1.1

## What were you trying to do

Trying to run tests as part of the package's build process.

## What happened

A test failed.

```
building
Building subPackage .
Building subPackage ./agessh
Building subPackage ./armor
Building subPackage ./cmd/age
Building subPackage ./cmd/age-keygen
Building subPackage ./internal/bech32
Building subPackage ./internal/format
Building subPackage ./internal/plugin
Building subPackage ./internal/stream
running tests
ok filippo.io/age 0.736s
ok filippo.io/age/agessh 0.272s
ok filippo.io/age/armor 0.016s
--- FAIL: TestScript (0.00s)
--- FAIL: TestScript/plugin (0.06s)
testscript.go:481:
# encrypt and decrypt a file with a test plugin (0.059s)
> age -r age1test10qdmzv9q -o test.age input
[stderr]
age: error: failed to wrap key for recipient #0: test plugin: write |1: broken pipe
age: report unexpected or unhelpful errors at https://filippo.io/age/report
[exit status 1]
FAIL: testdata/plugin.txt:2: unexpected command failure

FAIL
FAIL filippo.io/age/cmd/age 1.008s
FAIL
```

The build machine's CPU may have been overloaded at the time the tests were running (e.g. load average up to ~150 on a 16-core CPU), but it should not have caused an error AFAIK.