rustdoc: use current stage if download-rustc enabled

[onur-ozkan]

When using download-rustc, using stage 1 rustdoc results in the wrong librustc_driver being used.

$ ./build/host/stage1/bin/rustdoc --version
./build/host/stage1/bin/rustdoc: error while loading shared libraries: librustc_driver-7ff02ed05016d515.so: cannot open shared object file: No such file or directory

This change fixes that by not cutting the stage if download-rustc is enabled.

[rustbot]

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[onur-ozkan]

This is blocker for #122709.

[Kobzol]

r? @Kobzol

[Kobzol]

Confirmed that this fixes ./x build --stage 1 rustdoc with download-rustc = true.

We have discussed that rustdoc could eventually become a part of download-rustc = true, but this is useful as a fix in the meantime.

@bors r+

[bors]

📌 Commit 08fa5f0 has been approved by Kobzol

It is now in the queue for this repository.

[bors]

⌛ Testing commit 08fa5f0 with merge a4ce33c...

[bors]

☀️ Test successful - checks-actions
Approved by: Kobzol
Pushing a4ce33c to master...

[rust-timer]

Finished benchmarking commit (a4ce33c): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-8.0%	[-8.0%, -8.0%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

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

Cycles

Results (primary -0.8%)

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)	-	-	0
Improvements ✅ (primary)	-0.8%	[-0.8%, -0.8%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.8%	[-0.8%, -0.8%]	1

Binary size

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

Bootstrap: 695.971s -> 696.635s (0.10%)
Artifact size: 326.67 MiB -> 326.80 MiB (0.04%)