Avoid unnecessary builds/rebuilds of rust-demangler

[Zalathar]

This is a combination of two loosely-related changes:

• Don't build rust-demangler as a dependency of tests/run-make, because after Convert run-make/coverage-reports tests to use a custom compiletest mode #112300 none of the remaining run-make tests actually use it. (If future run-make tests ever do need the demangler, it'll be easy to add it back.)
• For tests/run-coverage, build the demangler with the stage 0 compiler instead of the current-stage compiler. This avoids having to uselessly rebuild the demangler after modifying and rebuilding the compiler itself.

[rustbot]

r? @onur-ozkan

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[onur-ozkan]

@bors r+ rollup=iffy

[bors]

📌 Commit 36c3f90 has been approved by onur-ozkan

It is now in the queue for this repository.

[bors]

⌛ Testing commit 36c3f90 with merge c6eb61a...

[bors]

☀️ Test successful - checks-actions
Approved by: onur-ozkan
Pushing c6eb61a to master...

[rust-timer]

Finished benchmarking commit (c6eb61a): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.3%	[1.3%, 1.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.3%	[1.3%, 1.3%]	1

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.9%	[2.4%, 3.7%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 635.428s -> 635.236s (-0.03%)
Artifact size: 304.56 MiB -> 304.58 MiB (0.01%)