Boost build emscripten toolset fails on Windows

**Version of emscripten/emsdk:**
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.13 (531257621816c200bc7c3be53129494afd029aec)
clang version 15.0.0 (https://github.com/llvm/llvm-project 5c6ed60c517c47b25b6b25d8ac3666d0e746b0c3)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: E:\emsdk\upstream\bin

**Failing command line in full:**
`b2.exe toolset=emscripten`

Output
```
E:\emsdk\python\3.9.2-nuget_64bit\python.exe: can't open file 'E:\boost\boost_1_78_0\emcc.py': [Errno 2] No such file or directory
E:/boost/boost_1_78_0/tools/build/src/build\property.jam:816: in class@property-map.find from module object(property-map)@39
error: Ambiguous key ms off pe boost off fcontext off on BOOST_ALL_NO_LIB=1 off on off true windows . off off off shared hidden OBJ off NT on on off off 3.8 address-model address-sanitizer architecture c++-template-depth cflags coverage cxxflags cxxstd cxxstd-dialect debug-symbols define define:toolset exception-handling flags force-include include inlining instruction-set leak-sanitizer link link:toolset local-visibility lto lto-mode optimization pch pch-file profiling rtti stdlib target-os thread-sanitizer threading toolset undefined-sanitizer warnings warnings-as-errors auto on on shared native off false project-relative @Jamfile%Jamfile.tag windows object(file-target)@7017 on win32 multi emscripten console debug off hidden off on boost.defaults=Boost
E:/boost/boost_1_78_0/tools/build/src/build\type.jam:340: in generated-target-ps from module type
E:/boost/boost_1_78_0/tools/build/src/build\type.jam:277: in type.generated-target-suffix from module type
E:/boost/boost_1_78_0/tools/build/src/build\virtual-target.jam:531: in virtual-target.add-prefix-and-suffix from module virtual-target
E:/boost/boost_1_78_0/tools/build/src/build\virtual-target.jam:490: in _adjust-name from module object(file-target)@7017
E:/boost/boost_1_78_0/tools/build/src/build\virtual-target.jam:287: in abstract-file-target.__init__ from module object(file-target)@7017
E:/boost/boost_1_78_0/tools/build/src/build\virtual-target.jam:583: in class@file-target.__init__ from module object(file-target)@7017
E:/boost/boost_1_78_0/tools/build/src/kernel\class.jam:90: in class.new from module class
E:/boost/boost_1_78_0/tools/build/src/build\generators.jam:582: in generated-targets from module object(C-compiling-generator)@136
E:/boost/boost_1_78_0/tools/build/src/build\generators.jam:468: in construct-result from module object(C-compiling-generator)@136
E:/boost/boost_1_78_0/tools/build/src/build\generators.jam:418: in run-really from module object(C-compiling-generator)@136
E:/boost/boost_1_78_0/tools/build/src/build\generators.jam:392: in class@generator.run from module object(C-compiling-generator)@136
E:/boost/boost_1_78_0/tools/build/src/build\generators.jam:1025: in try-one-generator-really from module generators
E:/boost/boost_1_78_0/tools/build/src/build\generators.jam:1087: in try-one-generator from module generators
E:/boost/boost_1_78_0/tools/build/src/build\generators.jam:1325: in construct-really from module generators
E:/boost/boost_1_78_0/tools/build/src/build\generators.jam:1411: in generators.construct from module generators
E:/boost/boost_1_78_0/tools/build/src/build\targets.jam:1635: in construct from module object(typed-target)@1379
E:/boost/boost_1_78_0/tools/build/src/build\targets.jam:1393: in class@basic-target.generate from module object(typed-target)@1379
E:/boost/boost_1_78_0/tools/build/src/build\targets.jam:812: in generate-really from module object(main-target)@6996
E:/boost/boost_1_78_0/tools/build/src/build\targets.jam:784: in class@main-target.generate from module object(main-target)@6996
E:/boost/boost_1_78_0/tools/build/src/build\targets.jam:934: in targets.generate-from-reference from module targets
E:/boost/boost_1_78_0/tools/build/src/build\configure.jam:389: in find-builds-raw from module configure
E:/boost/boost_1_78_0/tools/build/src/build\configure.jam:470: in configure.find-builds from module configure
E:/boost/boost_1_78_0\boostcpp.jam:618: in boostcpp.deduce-address-model from module boostcpp
E:/boost/boost_1_78_0/tools/build/src/kernel\modules.jam:106: in modules.call-in from module Jamfile
E:/boost/boost_1_78_0/tools/build/src/util\indirect.jam:105: in indirect.call from module indirect
E:/boost/boost_1_78_0/tools/build/src/build\property.jam:135: in property.evaluate-conditionals-in-context from module property
E:/boost/boost_1_78_0/tools/build/src/build\targets.jam:1088: in evaluate-requirements from module targets
E:/boost/boost_1_78_0/tools/build/src/build\targets.jam:1122: in common-properties2 from module targets
E:/boost/boost_1_78_0/tools/build/src/build\targets.jam:1018: in targets.common-properties from module targets
E:/boost/boost_1_78_0/tools/build/src/build\targets.jam:1314: in alias-target-class.generate from module object(top-level-target)@204
E:/boost/boost_1_78_0\boostcpp.jam:430: in build-multiple from module object(top-level-target)@204
E:/boost/boost_1_78_0\boostcpp.jam:420: in class@top-level-target.generate from module object(top-level-target)@204
E:/boost/boost_1_78_0/tools/build/src/build\targets.jam:812: in generate-really from module object(main-target)@6986
E:/boost/boost_1_78_0/tools/build/src/build\targets.jam:784: in class@main-target.generate from module object(main-target)@6986
E:/boost/boost_1_78_0/tools/build/src/build\targets.jam:273: in class@project-target.generate from module object(project-target)@147
E:/boost/boost_1_78_0/tools/build/src\build-system.jam:805: in load from module build-system
E:/boost/boost_1_78_0/tools/build/src/kernel\modules.jam:294: in import from module modules
E:/boost/boost_1_78_0/tools/build/src/kernel/bootstrap.jam:139: in boost-build from module
E:/boost/boost_1_78_0/boost-build.jam:17: in module scope from module
```