Fix: correct an error I made in PR7160

#### Brief overview of PR changes/additions
I made an error in Mudlet#7160 as I thought that something that looked like a
function call returned a value that could be assigned to something with a
`set(...)` but CMake doesn't work like that, instead one has to provide a
variable to recieve the value as the last argument to a `string(...)`
function. 🙄

#### Motivation for adding to Mudlet
Fix an error that causes cruft to appear, at least, in the version number
on the first tab of the "About Mudlet" dialogue.

#### Other info (issues closed, discussion etc)
This should close Mudlet#7220.

Signed-off-by: Stephen Lyons <slysven@virginmedia.com>

CMakeLists.txt

@@ -96,7 +96,7 @@ execute_process(
 # If a CI/CB build system provides an alternative Git SHA1 to identify the build
 # use that instead - reporting which is being used:
 if(DEFINED ENV{BUILD_COMMIT} AND NOT $ENV{BUILD_COMMIT} STREQUAL "")
-    set(GIT_SHA1 string(TOLOWER $ENV{BUILD_COMMIT}))
+    string(TOLOWER $ENV{BUILD_COMMIT} GIT_SHA1)
     message(STATUS "Git SHA1 set from environemnt: ${GIT_SHA1}")
 else()
     message(STATUS "Git SHA1 used: ${GIT_SHA1}")