fix: use canbench from HEAD in CI

[ielashi]

Problem

The CI is downloading a version of canbench from crates.io, which means the benchmarks do not capture any performance changes from the current PR.

Solution

Update the CI script to build canbench from HEAD.

Note the regression in the btreemap example due to the change in pricing of stable memory operations in later versions of canbench.

[github-actions]

canbench 🏋 (dir: examples/btreemap_vs_hashmap)

Significant performance change detected! ⚠️

examples/btreemap_vs_hashmap/canbench_results.yml is up to date ✅

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Benchmark: insert_users
  total:
    instructions: 2.57 B (no change)
    heap_increase: 871 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: remove_users
  total:
    instructions: 2.07 B (no change)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: pre_upgrade_bench
  total:
    instructions: 743.98 M (regressed by 2.04%)
    heap_increase: 519 pages (no change)
    stable_memory_increase: 184 pages (no change)

  serialize_state (scope):
    instructions: 729.04 M (-0.01%) (change within noise threshold)
    heap_increase: 519 pages (no change)
    stable_memory_increase: 0 pages (no change)

  writing_to_stable_memory (scope):
    instructions: 14.93 M (regressed by 2950599.60%)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 184 pages (no change)

---------------------------------------------------

[github-actions]

canbench 🏋 (dir: examples/fibonacci)

No significant performance changes detected ✅

examples/fibonacci/canbench_results.yml is up to date ✅

---------------------------------------------------

Benchmark: fibonacci_20
  total:
    instructions: 2224 (no change)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: fibonacci_45
  total:
    instructions: 3011 (no change)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------