build.bat: fix a pair of edge cases.

- don't expand PATH in comment to avoid hitting the line length limit;
- disambiguate link.exe invocation in case of interference from msys;

build.bat

@@ -1,6 +1,6 @@
 @echo off
 SETLOCAL
-set PATH=%windir%\system32;%PATH% &:: override msys if there is one on %PATH%
+set PATH=%windir%\system32;%PATH% &:: override msys if it's on the PATH
 set TOP=%~dp0
 cl /nologo /c /O2 /Zi /Fdblst.pdb /W4 /MT /Zl %TOP%src\server.c || EXIT /B
 cl 2>&1 | find "for ARM64" > nul:
@@ -18,7 +18,7 @@ IF ERRORLEVEL 1 (
 SETLOCAL ENABLEDELAYEDEXPANSION
 set static=/out:blst.lib
 set shared=
-set arm64ec=
+set arm64x=
 FOR %%O IN (%*) DO (
     set opt=%%O
     IF "!opt!" == "-shared" (
@@ -31,8 +31,15 @@ FOR %%O IN (%*) DO (
 )
 IF [%shared%] NEQ [] (
     cl /nologo /c /O2 /Oi- /MD %TOP%build\win64\dll.c || EXIT /B
+    set ld=
+    FOR /F "usebackq delims=" %%F IN (`where link`) DO (
+        IF "!ld!" == "" (
+            "%%F" 2>&1 | find "Linker" > nul:
+            IF !ERRORLEVEL! EQU 0 set ld="%%F"
+        )
+    )
     IF [%arm64x%] NEQ [yes] (
-        link /nologo /debug /dll /entry:DllMain /incremental:no %shared% ^
+        !ld! /nologo /debug /dll /entry:DllMain /incremental:no %shared% ^
              /def:%TOP%build\win64\blst.def *.obj kernel32.lib && del *.obj
     ) ELSE (
         lib /nologo /out:blst_arm64.lib *.obj && del *.obj || EXIT /B
@@ -41,7 +48,7 @@ IF [%shared%] NEQ [] (
             armasm64 -nologo -machine arm64ec -nowarn %%F || EXIT /B
         )
         cl /nologo /arm64EC /c /O2 /Oi- /MD %TOP%build\win64\dll.c || EXIT /B
-        link /nologo /machine:arm64x /dll /noentry %shared% ^
+        !ld! /nologo /machine:arm64x /dll /noentry %shared% ^
              /def:%TOP%build\win64\blst.def *.obj ^
              /defArm64Native:%TOP%build\win64\blst.def blst_arm64.lib ^
              kernel32.lib && del *.obj blst_arm64.lib