fix(ci): set VCToolsInstallDir as well — the two consumers do not read each other

Moving 14.51 out settled mcpp and broke clang. Each lever was measured
alone, and each failed in its own half:

  * `VCToolsInstallDir` alone — clang used 14.44's headers, mcpp kept
    scanning and handed it 14.51's std.ixx:
        std.ixx:126: fatal error: 'flat_map' file not found
  * moving 14.51 out alone — mcpp picked 14.44, and clang's own MSVC
    detection (which goes through the VS installation's registration, and
    that still names 14.51) found nothing. The compile came out with NO
    include paths at all:
        std.ixx:12: fatal error: 'assert.h' file not found
    `assert.h` is a UCRT header, and the command line carried neither
    /imsvc nor -I — clang was not looking in the wrong place, it had no
    place to look.

mcpp SCANS the toolset directory; clang READS the environment. Neither
reads the other's answer, so there is one lever per consumer and both are
needed. Setting both is not redundancy.

That took three rounds to land and the intermediate states are in the
step's comment, because each of them looks like a reasonable thing to try
and each is wrong in a way only the next failure shows.

Author: Sunrisepeak

.github/workflows/validate.yml

@@ -1049,6 +1049,25 @@ jobs:
           } else {
             Write-Host "14.51.36231 not present; nothing to move"
           }
+          # BOTH LEVERS, because they answer different consumers and each
+          # alone was measured failing:
+          #
+          #   * moving 14.51 out settles mcpp, which SCANS the directory and
+          #     takes the newest. Alone, it broke clang: clang's own MSVC
+          #     detection goes through the VS installation's registration,
+          #     which still names 14.51, so it found nothing and emitted a
+          #     compile with NO include paths at all --
+          #     `std.ixx:12: fatal error: 'assert.h' file not found`, the UCRT
+          #     header, from a command line carrying neither /imsvc nor -I.
+          #   * `VCToolsInstallDir` settles clang, which reads it instead of
+          #     detecting. Alone, it left mcpp scanning and picking 14.51, so
+          #     14.51's std.ixx compiled against 14.44's headers --
+          #     `std.ixx:126: fatal error: 'flat_map' file not found`.
+          #
+          # Neither consumer reads the other's answer. Setting both is not
+          # belt-and-braces; it is one answer per consumer.
+          "VCToolsInstallDir=$want\" | Out-File -Append -Encoding utf8 $env:GITHUB_ENV
+          Write-Host "VCToolsInstallDir -> $want"
           Write-Host "visible toolsets now:"
           Get-ChildItem $vs -Directory | ForEach-Object { Write-Host "  $($_.Name)" }
       - name: Download mcpp