Run bootstrap-vcpkg.bat when bootstrapped from git-bash (#6142)

* Bootstrap for windows when called from within bash for windows

When calling `bootstrap-vcpkg.sh` from within a git-bash script it fails
with the error message `Unknown uname: MINGW64_NT-10.0`.

This change delegates bootstrapping to `bootstrap-vcpkg.bat` by
executing it in a `cmd` session when the result of `uname -s` matches
the pattern `MINGW*_NT*`.

* [vcpkg-bootstrap] Avoid using bash extensions and keep ./bootstrap-vcpkg.sh as a simple forwarder.

scripts/bootstrap.sh

@@ -1,5 +1,20 @@
 #!/bin/sh
 
+# Find .vcpkg-root, which indicates the root of this repo
+vcpkgRootDir=$(X= cd -- "$(dirname -- "$0")" && pwd -P)
+while [ "$vcpkgRootDir" != "/" ] && ! [ -e "$vcpkgRootDir/.vcpkg-root" ]; do
+    vcpkgRootDir="$(dirname "$vcpkgRootDir")"
+done
+
+# Enable using this entry point on windows from git bash by redirecting to the .bat file.
+unixName=$(uname -s | sed 's/MINGW.*_NT.*/MINGW_NT/')
+if [ "$unixName" = "MINGW_NT" ]; then
+  vcpkgRootDir=$(cygpath -aw "$vcpkgRootDir")
+  cmd "/C $vcpkgRootDir\\bootstrap-vcpkg.bat" || exit 1
+  exit 0
+fi
+
+# Argument parsing
 vcpkgDisableMetrics="OFF"
 vcpkgUseSystem=false
 vcpkgAllowAppleClang=OFF
@@ -26,12 +41,6 @@ do
     fi
 done
 
-# Find vcpkg-root
-vcpkgRootDir=$(X= cd -- "$(dirname -- "$0")" && pwd -P)
-while [ "$vcpkgRootDir" != "/" ] && ! [ -e "$vcpkgRootDir/.vcpkg-root" ]; do
-    vcpkgRootDir="$(dirname "$vcpkgRootDir")"
-done
-
 if [ -z ${VCPKG_DOWNLOADS+x} ]; then
     downloadsDir="$vcpkgRootDir/downloads"
 else