Ignore function args when analysing expressions

[richfitz]

This PR changes how we analyse expressions, preventing the first case in the original ticket failing:

> id <- task_create_expr(
  expr = parallel::clusterApply(NULL, 1:2, function(x) {print(x)}),
  parallel = hipercow_parallel("parallel"),
  resources = resources)

Error in `rlang::env_get_list()`:
! Can't find `x` in environment.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (a924354) to head (25d5b2e).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #149   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        25           
  Lines         2411      2429   +18     
=========================================
+ Hits          2411      2429   +18     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests