chore: add the build script to the pretest script.

[lholmquist]

Currently, to run the tests successfully, npm run build needs to be run first in order to ajv compile the schema. We started to run into this when we added this library to the test suite that runs these tests on RHEL and Fedora platforms in our node.js container.

see sclorg/s2i-nodejs-container#384

error for reference:

TSError: ⨯ Unable to compile TypeScript:
src/event/spec.ts:10:22 - error TS2307: Cannot find module '../schema/v1' or its corresponding type declarations.

10 import validate from "../schema/v1";
                        ~~~~~~~~~~~~~~

    at createTSError (/Users/lholmqui/develop/cloudevents/sdk-javascript/node_modules/ts-node/src/index.ts:859:12)
    at reportTSError (/Users/lholmqui/develop/cloudevents/sdk-javascript/node_modules/ts-node/src/index.ts:863:19)
    at getOutput (/Users/lholmqui/develop/cloudevents/sdk-javascript/node_modules/ts-node/src/index.ts:1077:36)
    at Object.compile (/Users/lholmqui/develop/cloudevents/sdk-javascript/node_modules/ts-node/src/index.ts:1433:41)
    at Module.m._compile (/Users/lholmqui/develop/cloudevents/sdk-javascript/node_modules/ts-node/src/index.ts:1617:30)
    at Module._extensions..js (node:internal/modules/cjs/loader:1272:10)
    at Object.require.extensions.<computed> [as .ts] (/Users/lholmqui/develop/cloudevents/sdk-javascript/node_modules/ts-node/src/index.ts:1621:12)
    at Module.load (node:internal/modules/cjs/loader:1081:32)
    at Function.Module._load (node:internal/modules/cjs/loader:922:12)
    at Module.require (node:internal/modules/cjs/loader:1105:19)

[lance]

/lgtm