ci: make the e2e tests actually run

[someson]

Problem

The pty end-to-end tests guard themselves on prerequisites and skip when one is missing:

if bin == "" { t.Skip("bin/azform not built") }

CI never ran make build, so bin/azform never existed, so every e2e test skipped — and a skipped test reports as success. internal/ui finished in ~1.4s on CI instead of ~60s locally. The widget round trip has never been exercised on CI, including the two zsh tests that predate the bash work.

Changes

• Build the binary before testing, so the tests can run at all.
• Install zsh on ubuntu-latest, so the zsh tests have their prerequisite.
• AZFORM_E2E_REQUIRED=1 on ubuntu-latest turns any skip into a hard failure. macos-latest stays permissive: it ships bash 3.2, which genuinely cannot run the bash widget, so that skip is legitimate.

Unrelated bug found along the way

TestDoctorStateNoOverrides fails whenever any AZFORM_* variable is set — including AZFORM_NO_UPDATE_CHECK, which is documented in azform --help. A user who exports it cannot run go test ./... on a clean tree. This is on main today; the new env var merely exposed it.

Cause: t.Setenv(name, "") sets a variable to empty rather than removing it, so os.Environ() still lists NAME= and the doctor counts it as an override. Fixed with os.Unsetenv after t.Setenv.

Verification

Locally, simulating the ubuntu-latest configuration:

Check	Result
AZFORM_E2E_REQUIRED=1 go test -race ./...	all packages pass, internal/ui 65s
Guard off + binary missing	SKIP, run green
Guard on + binary missing	FAIL, loudly
AZFORM_NO_UPDATE_CHECK=1 go test ./cmd/azform/	now passes
golangci-lint run	0 issues

The 65s runtime is the signal that the e2e tests really ran.

What this PR is for

Confirming on a real Ubuntu runner that the e2e tests pass under the guard before this reaches main.