CI: Windows: downgrade rust to 1.77

It started failing CI as soon as the default shifted to 1.78. Something
is broken and it prevents running stable CI. Tracking issue opened.

We pin the version because that is the same way we handle CI for linux
-- with the exception that Linux CI can upgrade itself as soon as we
fix issues causing the CI Image Builder to jam itself, whereas
unfortunately Windows will need to be manually unpinned, but such is
life as a Windows supporter.

Bug: #13236

ci/run.ps1

@@ -8,21 +8,22 @@ if ($LastExitCode -ne 0) {
 $env:Path = ($env:Path.Split(';') | Where-Object { $_ -notmatch 'mingw|Strawberry|Chocolatey|PostgreSQL' }) -join ';'
 
 if ($env:arch -eq 'x64') {
+    rustup default 1.77
     # Rust puts its shared stdlib in a secret place, but it is needed to run tests.
-    $env:Path += ";$HOME/.rustup/toolchains/stable-x86_64-pc-windows-msvc/bin"
+    $env:Path += ";$HOME/.rustup/toolchains/1.77-x86_64-pc-windows-msvc/bin"
 } elseif ($env:arch -eq 'x86') {
     # Switch to the x86 Rust toolchain
-    rustup default stable-i686-pc-windows-msvc
-
-    # Also install clippy
-    rustup component add clippy
+    rustup default 1.77-i686-pc-windows-msvc
 
     # Rust puts its shared stdlib in a secret place, but it is needed to run tests.
-    $env:Path += ";$HOME/.rustup/toolchains/stable-i686-pc-windows-msvc/bin"
+    $env:Path += ";$HOME/.rustup/toolchains/1.77-i686-pc-windows-msvc/bin"
     # Need 32-bit Python for tests that need the Python dependency
     $env:Path = "C:\hostedtoolcache\windows\Python\3.7.9\x86;C:\hostedtoolcache\windows\Python\3.7.9\x86\Scripts;$env:Path"
 }
 
+# Also install clippy
+rustup component add clippy
+
 # Set the CI env var for the meson test framework
 $env:CI = '1'