fix: Use fresh instance on each new plugin invocation

[ospencer]

Porting this fix from suborbital/sat#179

[callahad]

@ospencer You're failing in CI:

--- FAIL: TestEngineFetch (0.27s)
panic: interface conversion: interface {} is *request.CoordinatedResponse, not []uint8 [recovered]
	panic: interface conversion: interface {} is *request.CoordinatedResponse, not []uint8

goroutine 31 [running]:
testing.tRunner.func1.2({0x13c9ce0, 0xc0012d0060})
	/opt/hostedtoolcache/go/1.20.2/x64/src/testing/testing.go:1526 +0x24e
testing.tRunner.func1()
	/opt/hostedtoolcache/go/1.20.2/x64/src/testing/testing.go:1529 +0x39f
panic({0x13c9ce0, 0xc0012d0060})
	/opt/hostedtoolcache/go/1.20.2/x64/src/runtime/panic.go:884 +0x213
github.com/suborbital/e2core/sat/engine2/enginetest.TestEngineFetch(0xc000170680)
	/home/runner/work/e2core/e2core/sat/engine2/enginetest/engine_test.go:159 +0x758
testing.tRunner(0xc000170680, 0x1450e80)
	/opt/hostedtoolcache/go/1.20.2/x64/src/testing/testing.go:[157](https://github.com/suborbital/e2core/actions/runs/4619606359/jobs/8168575352?pr=413#step:7:158)6 +0x10b
created by testing.(*T).Run
	/opt/hostedtoolcache/go/1.20.2/x64/src/testing/testing.go:[162](https://github.com/suborbital/e2core/actions/runs/4619606359/jobs/8168575352?pr=413#step:7:163)9 +0x3ea
FAIL	github.com/suborbital/e2core/sat/engine2/enginetest	0.484s

[callahad]

@javorszky Any chance you can figure out the CI failure?

[callahad]

Other recent PRs also show the same error, so this PR isn't the cause. We should review + merge independently of diagnosing that test failure.

[callahad]

Tracking the CI failure in #414

[javorszky]

the fun thing is that the same issue does not appear on my local when I run make test.

When running the one test function from inside goland, it does happen though. Weird.

[javorszky]

Made two small changes on top of what the original PR was:

• skipping a test that depends on external something outside of our control (1password error page)
• passing in instance into deferred function and using a differently named variable inside the function to better separate scopes and guard against shadowing