chore: fix the guest compiler's rust toolchain version

[jonathanpwang]

We must use nightly for -Z but we fix a nightly version for stability.

[github-actions]

Benchmarks

group	app_log_blowup	app_total_cells_used	app_total_cycles	app_total_proof_time_ms	leaf_log_blowup	leaf_total_cells_used	leaf_total_cycles	leaf_total_proof_time_ms	instance	alloc
base64_json_program	2	(-15,402 [-0.1%]) 15,111,492	(-6 [-0.0%]) 217,347	(+24.0 [+0.9%]) 2,690.0	-	-	-	-	64cpu-linux-arm64	mimalloc
ecrecover_program	2	(-2,865,108 [-1.0%]) 284,010,873	(-69,693 [-1.3%]) 5,163,156	(-225.0 [-0.8%]) 26,467.0	-	-	-	-	64cpu-linux-arm64	mimalloc
fibonacci_program	2	(-1,105 [-0.0%]) 51,645,731	1,500,219	(-44.0 [-0.7%]) 6,648.0	-	-	-	-	64cpu-linux-arm64	mimalloc
fibonacci_program	2	(+1,249 [+0.0%]) 51,645,721	1,500,219	(-232.0 [-3.2%]) 7,037.0	-	-	-	-	64cpu-linux-x64	jemalloc
regex_program	2	(+148,125 [+0.1%]) 238,973,705	(+9,561 [+0.2%]) 4,190,904	(-13.0 [-0.0%]) 27,238.0	-	-	-	-	64cpu-linux-arm64	mimalloc
verify_fibair	2	(-40 [-0.0%]) 48,127,085	(-25 [-0.0%]) 198,580	(+9.0 [+0.2%]) 5,667.0	-	-	-	-	64cpu-linux-arm64	mimalloc

Commit: bc82b6d

Benchmark Workflow