rustbuild: Fix handling of the bootstrap key

[alexcrichton]

Bring the calculation logic in line with the makefiles and also set the
RUSTC_BOOTSTRAP_KEY environment variable to enable the bootstrap on the stable
compiler.

[brson]

@bors r+

[bors]

📌 Commit 8efbfae has been approved by brson

[bors]

⌛ Testing commit 8efbfae with merge 66d0a28...

[bors]

💔 Test failed - auto-win-gnu-32-opt-rustbuild

[alexcrichton]

@bors: retry

On Thu, Apr 14, 2016 at 8:19 AM, bors [email protected] wrote:

[image: 💔] Test failed - auto-win-gnu-32-opt-rustbuild
http://buildbot.rust-lang.org/builders/auto-win-gnu-32-opt-rustbuild/builds/681

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#32895 (comment)

[bors]

⌛ Testing commit 8efbfae with merge 3c41f9d...

[bors]

💔 Test failed - auto-win-gnu-32-opt

[alexcrichton]

@bors: retry

On Thu, Apr 14, 2016 at 12:05 PM, bors [email protected] wrote:

[image: 💔] Test failed - auto-win-gnu-32-opt
http://buildbot.rust-lang.org/builders/auto-win-gnu-32-opt/builds/3782

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#32895 (comment)

[bors]

⌛ Testing commit 8efbfae with merge 74b3684...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-64-x-armhf, auto-linux-64-x-armsf, auto-linux-64-x-freebsd, auto-linux-64-x-netbsd, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-cross-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild