[vcpkg] add environment variable VCPKG_OVERLAY_PORTS

[Neumann-A]

No description provided.

[JackBoosY]

Provide us some usage?

[qis]

@JackBoosY Here is some usage. It seems to work as intended.

set VCPKG_OVERLAY_PORTS=C:\Workspace\a;C:\Workspace\b
vcpkg install test
rem Installs: C:\Workspace\a\test

set VCPKG_OVERLAY_PORTS=C:\Workspace\b;C:\Workspace\a
vcpkg install test
rem Installs: C:\Workspace\b\test

export VCPKG_OVERLAY_PORTS=/mnt/c/Workspace/a:/mnt/c/Workspace/b
vcpkg install test
# Installs: /mnt/c/Workspace/a

export VCPKG_OVERLAY_PORTS=/mnt/c/Workspace/b:/mnt/c/Workspace/a
vcpkg install test
# Installs: /mnt/c/Workspace/b

Edit: Overriding directories in vcpkg/ports is also possible.

[qis]

Adding this might be a good idea:

diff --git i/docs/users/config-environment.md w/docs/users/config-environment.md
index ffa988cbc..db9d26ac3 100644
--- i/docs/users/config-environment.md
+++ w/docs/users/config-environment.md
@@ -38,11 +38,19 @@ Example: `D:\2017`
 
 This environment variable can be set to a triplet name which will be used for unqualified triplet references in command lines.
 
+#### VCPKG_OVERLAY_PORTS
+
+This environment variable allow users to override ports with alternate versions according to the
+[ports overlay](../specifications/ports-overlay.md) specification. Use `;` as a delimitor on Windows
+and `:` on all other systems.
+
+Example: `C:\custom-ports\boost;C:\custom-ports\sqlite3`
+
 #### VCPKG_FORCE_SYSTEM_BINARIES
 
 This environment variable, if set, suppresses the downloading of CMake and Ninja and forces the use of the system binaries.
 
-### VCPKG_KEEP_ENV_VARS
+#### VCPKG_KEEP_ENV_VARS
 
 This environment variable can be set to a list of environment variables, separated by `;`, which will be propagated to
 the build environment.

[strega-nil]

I really like this feature.

[strega-nil]

cc @ras0219-msft

[Neumann-A]

@strega-nil: probably should follow up with another one for VCPKG_OVERLAY_TRIPLETS. But that one is not so important as this one.

[JackBoosY]

pbc regression will be fixed in #12732