test: retain process.env when overriding NETLIFY vars (#8412)

The previous code was nuking the entire `process.env` which meant some
built-in tools like `os.tmpdir()` would be `undefined` on Windows (as it
depends on `env.TEMP` or some such thing existing).

We should be able to safely extend the existing env instead of
overwriting it.

I suspect this will fix:

https://github.com/netlify/cli/actions/runs/31727425876/job/94549735809#step:10:286

Since that error happens when joining an `undefined` tmpdir.

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Author: 43081j

tests/integration/commands/build/build-program.test.ts

@@ -71,7 +71,7 @@ describe('command/build', () => {
       // eslint-disable-next-line no-restricted-properties
       process.cwd = () => builder.directory
       await withMockApi(routes, async ({ apiUrl }) => {
-        process.env = getEnvironmentVariables({ apiUrl })
+        process.env = { ...originalEnv, ...getEnvironmentVariables({ apiUrl }) }
 
         await builder.withNetlifyToml({ config: {} }).withStateFile({ siteId: siteInfo.id }).build()