runtime-env tests fail on Windows without sh, and one passes for the wrong reason

[ndreno]

ui/src/lib/__tests__/runtime-env.test.ts runs docker/env.sh, the script the images use to generate env.js at startup, through execFileSync("sh", [script], ...). On Windows without an sh on the PATH, npm test in ui/ fails:

• writes valid JavaScript carrying the settings and defaults to no OIDC and no local login form fail with spawnSync sh ENOENT.
• fails rather than serving no configuration when it cannot write passes for the wrong reason: it expects execFileSync to throw, and the missing sh throws too, so the test says nothing about the script on Windows.

Reported by a contributor in #27 (168/170 passing on Windows).

Proposed fix

1. Skip the suite when no POSIX shell is available, for example with describe.skipIf(...) after checking spawnSync("sh", ["-c", "true"]) for an ENOENT error. The script only runs inside Linux images, so there is nothing to test without sh.
2. In the failure test, assert that the script itself failed: a non-zero exit status (error.status), not a spawn error (error.code === "ENOENT").

Done when

• npm test in ui/ passes on Windows without Git Bash, with this suite reported as skipped.
• On Linux and macOS the three tests still run, and the failure test fails if the script is changed to exit 0 when it cannot write.